Phase Transition Of Vortex Matter Research Articles

Inhomogeneity and irreversibility field of superconducting Nb-Ti tapes

Magnetizations and current-voltage characteristics of superconducting tapes from Nb-50% wt.Ti alloy are studied. In the region of external magnetic fields close to the upper critical field, there is a so called irreversibility field, which is often associated with the phase transition of vortex matter. However, in this study it is shown that the whole volume of experimental data can be described if we abandon the hypothesis of a phase transition and go over to the hypothesis of macroscopic inhomogeneity. This hypothesis implies the existence of two superconducting components with different upper critical fields, which naturally arise as a result of the production process.

Angular dependence of vortex matter phase transition in MgB 2 single crystal

The vortex matter phase transitions and intrinsic pinning effect were investigated in an MgB 2 single crystal using the torque magnetometry. For the field directions apart from the ab plane, we succeed in the observation of the vortex lattice melting transitions, which are transformed from the order–disorder transitions at low temperatures. Both transition fields with field directions can be describe by the GL effective mass model. For the field direction along the ab plane, these transitions become unobservable. Instead, the sudden increase in the hysteresis of magnetization curve occurs, indicating the existence of the intrinsic pinning coming from the layer structure.

Vortex matter driven through mesoscopic channels

The dynamics of vortex matter confined to mesoscopic channels has been investigated by means of mode locking experiments. When vortices are coherently driven through the potential provided by static vortices pinned in the channel edges, interference between the washboard frequency of the moving vortex lattice and the frequency of the superimposed rf-drive causes (Shapiro-like) steps in the dc- I– V curves. The position of the voltage steps uniquely determines the number of moving rows in each channel. It also shows how the frustration between row spacing and channel width behaves as a function of magnetic field. Maxima in flow stress (∼ I c) occur at mismatch conditions. They are related to the traffic-jam-like flow impedance caused by the disorder in the edges. At higher fields, near the 2D-melting line B m( T), the mode-locking interference characteristic for crystalline motion, strongly depends on the velocity, i.e. the applied frequency at which the vortex motion is probed. The minimum velocity at which coherent motion could be observed, diverges when the melting line is approached from below. Above the melting line interference is absent for any frequency. These observations give the first direct evidence for a dynamic phase transition of vortex matter driven through a disorder potential as predicted by Koshelev and Vinokur.

Disorder-induced phase transition of vortex matter inMgB2

Measurements of single crystal ${\mathrm{MgB}}_{2}$ with torque magnetometry in fields up to 90 kOe reveal a sharp peak in the irreversible torque at about $0.85{H}_{c2}.$ In the region between peak onset and maximum, pronounced history effects occur. Angle and temperature dependence of the characteristic peak fields track those of ${H}_{c2}.$ The features observed suggest that the peak marks a disorder-induced phase transition of vortex matter between a quasiordered Bragg glass and a highly disordered phase.

MgB2 single crystals: high pressure growth and physical properties

Single crystals of MgB2 with a size up to 1.5 × 0.9 × 0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ∼0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H//cc2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H//abc2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H//abc2/H//cc2 was found to be temperature dependent (decreasing from γ ≅ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γeff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from γeff ≅ 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H//cc2.

Anisotropy of the superconducting state properties and phase diagram of MgB 2 by torque magnetometry on single crystals

The angular and temperature dependence of the upper critical field H c2 in MgB 2 was determined from torque magnetometry measurements on single crystals. The H c2 anisotropy γ H was found to decrease with increasing temperature, in disagreement with the anisotropic Ginzburg–Landau theory, which predicts that γ H is temperature independent. This behaviour can be explained by the two band nature of superconductivity in MgB 2. An analysis of measurements of the reversible torque in the mixed state yields a field dependent effective anisotropy γ eff, which can be at least partially explained by different anisotropies of the penetration depth and the upper critical field. It is shown that a peak effect in fields of about 0.85 H c2 is a manifestation of an order–disorder phase transition of vortex matter. The H– T phase diagram of MgB 2 for H∥ c correlates with the intermediate strength of thermal fluctuations in MgB 2, as compared to those in high and low T c superconductors.

Hysteretic behavior of the vortex lattice at the onset of the second peak for theHgBa2CuO4+δsuperconductor

By means of local Hall probe ac and dc permeability measurements we investigated the phase diagram of vortex matter for the HgBa$_2$CuO$_{4+\delta }$ superconductor in the regime near the critical temperature. The second peak line, $H_{\rm sp}$, in contrast to what is usually assumed, doesn't terminate at the critical temperature. Our local ac permeability measurements revealed pronounced hysteretic behavior and thermomagnetic history effects near the onset of the second peak, giving evidence for a phase transition of vortex matter from an ordered qausilattice state to a disordered glass.

First-order disorder-driven transition and inverse melting of the vortex lattice

Vortex matter phase transitions in the high-temperature superconductor Bi 2Sr 2CaCu 2O 8 were studied using local magnetization measurements combined with a vortex `shaking' technique. The measurements revealed thermodynamic evidence of a first-order transition (FOT) along the second magnetization peak line, at temperatures below the apparent critical point T cp. We found that the FOT line does not terminate at T cp, but continues down to at least 30 K. This observation suggests that the ordered vortex lattice phase is destroyed through a unified FOT that changes its character from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures. At intermediate temperatures the transition line shows an upturn, which implies that the vortex matter displays `inverse' melting behavior.

Vortex matter phase transitions under tilted fields in pristine and Pb-substituted Bi 2Sr 2CaCu 2O 8+ y

Vortex matter phase transitions in Bi 2Sr 2CaCu 2O 8+ y (BSCCO) under fields tilted from the c-axis is studied by using Hall probes. Vortex lattice melting ( H m (θ)) and peak effect ( H p (θ)) are found to follow a novel angular scaling; transition fields linearly depends on the in-plane component of the field. The novel scaling is valid in the vortex solid phase up to doping-level-dependent angle, θ*. New anomalies in the magnetization hysteresis curve, H* p and H* cp , appear only at large tilting angles. Above θ*, the angular dependence of H p , H* p , and H* cp changes drastically, suggesting the presence of vortex phase transition at this angle.

Effect of Bi–Sr replacement and oxygen doping on vortex-matter phase transitions in Bi 2+ xSr 2− xCaCu 2O 8+ δ

To investigate the effect of oxygen doping level with a wide range on vortex-matter phase transitions, magnetization for H‖ c was measured for Bi 2+ x Sr 2− x CaCu 2O 8+ δ single crystal with x=0.05–0.25 and various doping level δ. Optimum condition against δ for the superconducting transition temperature was found to depend on x, which is reasonably explained from the difference in valences between Bi and Sr. The second-peak of the low temperature magnetization curve was observed in all the samples, ranging from ∼120 Oe for the mostly underdoped sample to ∼1300 Oe for the most overdoped one. For the optimally-doped samples the second-peak was observed at ∼400 Oe almost independent of x. Vortex-melt transition field, H m, was well evaluated for the as-grown samples and for the slightly overdoped ones. The shift of the melting line, H m( T), with x and δ can be explained from the change in anisotropy and penetration depth. Near the superconducting transition temperature vortex transition is attributed to the transition due to the decoupling of pancake vortices, while at lower temperatures to the melting transition with both the electromagnetic- and the Josephson-coupling between vortices.

Vortex matter phase transitions in Bi 2Sr 2CaCu 2O 8

Vortex-lattice phase transitions in Bi 2Sr 2CaCu 2O 8 crystals are studied using local magnetization measurements. The vortex matter is found to exhibit at least three distinct phases. A rather ordered quasilattice phase is present at low fields. At elevated temperatures the quasilattice melts (or sublimates) through a first-order phase transition, whereas at lower temperatures a transition to a highly disordered vortex solid occurs at the second magnetization peak. A very low density of columnar defects results in significant pinning of the quasilattice, demonstrating the finite shear modulus of this phase. Local ac measurements reveal that in the presence of columns the pinned quasilattice still melts through a first-order phase transition with an observable step in equilibrium magnetization at elevated temperatures.