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1.
We studied the commensurate semifluxon oscillations of Josephson flux-flow in Bi-2212 stacked structures near Tc as a probe of melting of a Josephson vortex lattice. We found that oscillations exist above 0.5 T. The amplitude of the oscillations is found to decrease gradually with the temperature and to turn to zero without any jump at T = T0 (3.5 K below the resistive transition temperature Tc), thus, indicating a phase transition of the second order. This characteristic temperature T0 is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, TBKT, in the elementary superconducting layers of Bi-2212 at zero magnetic field. On the basis of these facts, we infer that melting of a triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and antivortices. The B-T phase diagram of the BKT phase found from our experiment is consistent with theoretical predictions.  相似文献   

2.
Temperature dependences of the resistivity ρ(T) of samples of granular high-temperature superconductor YBa2Cu3O7 – δ are measured at various transverse external magnetic fields at 0 < H ext < 1900 Оe in the temperature range from the upper Josephson critical temperature of “weak bonds” T c2J to temperatures slightly exceeding the superconducting transition temperature T c . Based on the data obtained, the behavior of the field dependences of the critical temperatures of superconducting grains and “weak bonds,” and temperature and field dependences of the magnetic contribution to the resistivity \(\left[ {\Delta \rho \left( {T,H} \right) = \rho {{\left( T \right)}_{{H_{ext}} = const}} - \rho {{\left( T \right)}_{{H_{ext}} = 0}}} \right]\). It is shown that the behavior of the magnetic contribution to the resistivity Δρ along the line of the phase transition related to the onset of the magnetic field penetration in the form of Abrikosov vortices into the subsystem of superconducting grains T c1g (H ext) is anomalous. The concepts on the magnetic flux redistribution between both subsystems of two-level HTSC near in the vicinity of T c1g : the Josephson vortex decreases, and the Abrikosov vortex density increases.  相似文献   

3.
The behavior in a magnetic field of a highly correlated electron liquid approaching the fermion condensation quantum phase transition from the disordered phase is considered. We show that, at sufficiently high temperatures TT*(x), the effective mass starts to depend on T, M* ∝T?1/2. This T?1/2 dependence of the effective mass at elevated temperatures leads to the non-Fermi liquid behavior of the resistivity, σ(T) ∝ T and at higher temperatures σ(T) ∝ T3/2. The application of a magnetic field B restores the common T2 behavior of the resistivity. The effective mass depends on the magnetic field, M*(B) ∝ B?2/3, being approximately independent of the temperature at T≤T*(B) ∝ B4/3. At TT*(B), the T?1/2 dependence of the effective mass is reestablished. We demonstrate that this B-T phase diagram has a strong impact on the magnetoresistance (MR) of the highly correlated electron liquid. The MR as a function of the temperature exhibits a transition from negative values of MR at T→0 to positive values at TB4/3. Thus, at TT*(B), MR as a function of the temperature possesses a node at TB4/3.  相似文献   

4.
Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature Tc of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers ds is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature Tc at the reduction of the thickness ds is detected and is attributed to the 0-π transition.  相似文献   

5.
The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS3 on the direction and magnitude of the magnetic field B have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at T < 100 K. Below T0 ≈ 50 K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance (Bc) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at T0. The negative magnetoresistance (at Bc) below T0 is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at Bab) is explained by the effect of the magnetic field on the spectrum of electronic states.  相似文献   

6.
The superconducting-insulator transition is simulated in disordered networks of Josephson junctions with thermally activated Arrhenius-like resistive shunt. By solving the conductance matrix of the network, the transition is reproduced in different experimental conditions by tuning thickness, charge density and disorder degree. In particular, on increasing fluctuations of the parameters entering the Josephson coupling and the Coulomb energy of the junctions, the transition occurs for decreasing values of the critical temperature T c and increasing values of the activation temperature T o . The results of the simulation compare well with recent experiments where the mesoscopic fluctuations of the phase have been suggested as the mechanism underlying the phenomenon of emergent granularityin otherwise homogeneous films. The proposed approach is compared with the results obtained on TiN films and nanopatterned arrays of weak-links, where the superconductor-insulator transition is directly stimulated.  相似文献   

7.
8.
In QCD, the strengths of the large scale temperature dependent chromomagnetic, B3, B8, and usual magnetic, H fields spontaneously generated in quark-gluon plasma after the deconfinement phase transition (DPT), are estimated. The consistent at high temperature effective potential accounting for the oneloop plus daisy diagrams is used. The heavy ion collisions at the LHC and temperatures T not much higher than the phase transition temperature Td are considered. The critical temperature for the magnetized plasma is found to be Td (H) ~ 110–120 MeV. This is essentially lower compared to the zero field value Td (H=0) ~ 160–180 MeV usually discussed in the literature. Due to contribution of quarks, the color magnetic fields act as the sources generating H. The strengths of the fields are B3(T), B8(T) ~ 1018–1019 G, H(T) ~ 1016–1017 G for temperatures T ~ 160–220 MeV. At temperatures T < 110–120 MeV the effective potential minimum value being negative approaches to zero. This is signaling the absence of the background fields and color confinement.  相似文献   

9.
The following parameters have been obtained for a thick (thickness t ~50 μm) film of high-temperature superconducting ceramics of the Bi-2223 system have been implemented: magnetosensitivity S u ~ 29 V T?1 and resolutions δB ~ 3 nT and δ? ~ 4?0 in magnetic field and magnetic flux, respectively. It is shown that the film magnetosensitivity can be significantly increased due to the size effect. The expected characteristics, estimated with allowance for the size effect, are S u ≥ 1000 V T?1, δB ~ 0.01 nT, δ? ~ 0.01 ?0, and the range of dynamic measurement ≥150 dB.  相似文献   

10.
Experimental studies of the antiferromagnetic (AF) heavy fermion metal YbRh2Si2 in a magnetic field B indicate the presence of a jump in the Hall coefficient at a magnetic-field tuned quantum state in the zero temperature limit. This quantum state occurs at BBc0 and induces the jump even though the change of the magnetic field at B = Bc0 is infinitesimal. We investigated this by using the model of heavy electron liquid with the fermion condensate. Within this model, the jump takes place when the magnetic field reaches the critical value Bc0 at which the ordering temperature TN(B = Bc0) of the AF transition vanishes. We show that at BBc0, this second order AF phase transition becomes the first order one, making the corresponding quantum and thermal critical fluctuations vanish at the jump. At T → 0 and B = Bc0 the Grüneisen ratio as a function of the temperature T diverges. We demonstrate that both the divergence and the jump are determined by the specific low temperature behavior of the entropy \(S(T) \propto S_0 + a\sqrt T + bT\) with S0; a and b are temperature independent constants.  相似文献   

11.
The upper field of the Meissner regime, H up, and overheat field Hc1, above which vortices start penetrating into a Josephson contact, are calculated throughout the range of pinning parameter I. The stability of likely configurations is investigated. It is shown that H up = Hc1 at any I. The existence of a single vortex centered at the extreme cell in the contact is demonstrated to be a possibility. At I > 3.69, such a vortex may exist even in a zero magnetic field. At 1.48 < I < 3.69, this vortex can exist in an external field in the range from some H v to H up. At I < 1.48, the vortex cannot exist under any conditions. From the equality of H up and Hc1 at any I, the conclusion is drawn that penetration of vortices into any Josephson medium is conditioned by the need to satisfy flux quantization conditions. Here, not the forces of vortex pinning at defects in the medium but quantization requirements are of major importance, which are satisfied in specific quantum ways rather than by meeting equilibrium conditions for vortices, forces, etc.  相似文献   

12.
All four parameters of the Mie–Lennard-Jones pair interatomic potential have been determined, and the state equation (P) and baric dependences of the lattice properties of an fcc iron are calculated using a previously proposed method. The dependences have been studied for the following properties: Debye temperature; the first, second, and third Gruneisen parameters; isothermal bulk modulus B T and B′(P); isochoric specific heat C v and C v ′(P); isobaric specific heat C p ; coefficient of thermal expansion α p and α p ′(P); specific surface energy σ and σ′(P). Calculations performed along two isotherms (1500 and 3000 K) have shown good agreement with the experimental data. Analytical approximations of the baric dependences for B′(P), α p (P), C p (P), and σ′(P) have been obtained, and it is shown that at P → ∞ the functions B T (P) and σ(P) change linearly, while the functions α p(P) and C p ′(P) tend to zero. The calculated baric dependence of the melting temperature shows good agreement with the experimental data.  相似文献   

13.
The effect of heating and cooling rates on melting (Tm) and crystallization (Tc) temperatures of metal nanoclusters is investigated in terms of the isothermal molecular dynamics. We report on the results obtained for nickel nanoclusters, although analogous results were also obtained for gold and aluminum nanoclusters. It is found that Tm increases, while Tc decreases with increasing heating and cooling rates, both Tm and Tc tending to the same value for heating and cooling rates tending to zero. The results indicate that the hysteresis of melting and crystallization of nanoparticles must be completely due to nonequilibrium conditions of heating and cooling. The transition of Ni nanoclusters to the amorphous state begins at very high cooling rates exceeding 10 TK/s.  相似文献   

14.
The main properties and the type of the field-tuned quantum critical point in the heavy-fermion metal CeCoIn5 that arise upon application of magnetic fields B are considered within a scenario based on fermion condensation quantum phase transition. We analyze the behavior of the effective mass, resistivity, specific heat, charge, and heat transport as functions of applied magnetic fields B and show that, in the Landau Fermi liquid regime, these quantities demonstrate critical behavior, which is scaled by the critical behavior of the effective mass. We show that, in the high-field non-Fermi liquid regime, the effective mass exhibits very specific behavior, M*~ T? 2/3, and the resistivity demonstrates T2/3 dependence. Finally, at elevated temperatures, it changes to M*~T?1/2, while the resistivity becomes linear in T. In zero magnetic field, the effective mass is controlled by temperature T and the resistivity is also linear in T. The obtained results are in good agreement with recent experimental facts.  相似文献   

15.
Modulation instability of dispersive electromagnetic waves propagating through a Josephson junction in a thin superconducting film is investigated in the framework of the nonlocal Josephson electrodynamics. A dispersion relation is found for the time increment of small perturbations of the amplitude. For dispersive waves, it is first established that spatial nonlocality suppresses the modulation instability in the range of perturbation wave vectors 0≤QQB1(k), i.e., in the long-wavelength range of experimental interest. The modulation instability range QB1(k)<Q<QB2(k, A, L) can be controlled (which is a unique possibility) by varying a dispersion parameter, namely, the wave vector k [or the frequency ω(k)] of linear-approximation waves. In the wave-vector ranges 0≤QQB1(k) and QQB2(k, A, L), waves are shown to be stable.  相似文献   

16.
The melting and growth of3He crystals, spin-polarized by an external magnetic field, are different in nature depending on whether the temperature is higher or lower than the characteristic ordering temperatures in the crystal (the Neel temperatureT N ) and in the liquid (the superfluid transition temperatureT c ). In the high-temperature region (T≥T N ,T c ) the liquid which appears upon melting has a high nonequilibrium spin density. In the low-temperature region (T?T N ,T c ) the melting and growth are accompanied by spin supercurrents both in the liquid and in the crystal in addition to mass supercurrents in the liquid. The crystallization waves at the liquid-solid interface should exist in the low-temperature region. With increasing magnetic field the waves change in nature, because the spin currents begin to play a dominant role. The wave spectrum becomes linear with a velocity inversely proportional to the magnetic field. The attenuation of the waves at low enough temperatures is mainly due to the interaction of the moving crystal-liquid interface with thermal spin waves in the crystal. The waves could be weakly damped at temperatures below a few hundreds microkelvins.  相似文献   

17.
The magnetoconductivity σ(B) of a classical 2D electron crystal on superfluid4He is non-linear. Experimentally we find a contribution to σ(B) which at constant field, gives σ(B)∞J x, the current density, while at constant current, σ(B) ∞ 1/B. In this region the Hall velocity νH slowly approaches the ripplon velocity νI at the first reciprocal lattice vector, due to strong electron-ripplon interactions with the helium. The magnetoconductivity decreases sharply for νHI. Fluctuations in σ(B) are seen above the melting temperature.  相似文献   

18.
The spectrum of magnetoelastic waves in a periodic structure of alternating ferromagnetic and nonmagnetic layers was studied. In the case of ferromagnetic layers with easy magnetization axes parallel to the layer surfaces, an orientational phase transition induced by an external tangential magnetic field He was considered. The formation of an inhomogeneous phase with a spatially modulated order parameter, which is caused by the magnetization being coupled through magnetostriction to lattice strains near the interfaces separating the magnetoelastic from elastic media, is predicted. It is shown that at a certain critical field in excess of the orientational phase transition field in the system without magnetostriction, a magnetoelastic wave propagating in a direction parallel to the in-plane magnetization vector M becomes unstable at finite values of the wave vector and condenses into a magnetostriction domain structure. A phase diagram in the (L, T, He) coordinates is constructed, and the regions of existence of thermodynamically equilibrium collinear, canted, and domain phases are established (L and T are the thicknesses of the ferromagnetic and nonmagnetic layers, respectively).  相似文献   

19.
Using the dependences of melting point Tm and crystallization point Tc on the number of atoms (N) in a spherical silicon crystal that were calculated elsewhere [6] by the method of molecular dynamics, (i) the number of atoms at which the latent heat of the solid–liquid phase transition disappears and (ii) temperature T0 = Tm(N0) = Tc(N0) below which solidifying nanoclusters remain noncrystalline are estimated. These values are found to be N0 = 22.8156 and T0 = 400.851 K. The N dependences for silicon melting parameters, namely, a jump of entropy of melting, latent melting heat, slope of the melting line, and jumps in the surface energy and volume, are derived.  相似文献   

20.
We discretize the Weyl equation for a massless, spin-1/2 particle on a time-diagonal, hypercubic spacetime lattice with null faces. The amplitude for a step of right-handed chirality is proportional to the spin projection operator in the step direction, while for left-handed it is the orthogonal projector. Iteration yields a path integral for the retarded propagator, with matrix path amplitude proportional to the product of projection operators. This assigns the amplitude i ±T 3?B/2 2?N to a path with N steps, B bends, and T right-handed minus left-handed bends, where the sign corresponds to the chirality. Fermion doubling does not occur in this discrete scheme. A Dirac mass m introduces the amplitude i ?? m to flip chirality in any given time step ??, and a Majorana mass similarly introduces a charge conjugation amplitude.  相似文献   

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