Muon Spin Rotation Studies of the Vortex Matter in the High-T c Superconductor Bi 2 Sr 2 CaCu 2 O 8+δ

We report on magnetic and magnetoresistance measurements in two categories of superconducting Nb films grown via magnetron sputtering and MgB−2 bulk samples. In the first category, films of Tc = 9.25 K were produced by annealing during deposition. In these films, the magnetic measurements exhibited the so-called "second magnetization peak" ("SMP"), which is accompanied by thermomagnetic instabilities (TMI). The characteristic field H fj , where the first flux jump occurs, has been studied as a function of the sweep rate of the magnetic field. Interestingly, in the regime T < 6.4 K, the respective line H fj (T ) is constant, H fj (T< 6.4 K)= 40 Oe. A comparison to TMI observed in MgB2 bulk samples is also performed. Our experimental findings can't be described accurately by current theories on TMI. In the second category, films of Tc = 8.3 K were produced without annealing during deposition. In such films, we observed a peak effect (PE). In high magnetic fields the PE is accompanied by a sharp drop and a narrow hysteretic behavior (∆T < 20 mK) in the measured magnetoresistance. In contrast to experimental works presented in the past, the comparison of our magnetic measurements with the magnetoresistance data suggests that rather the appearance of surface superconductivity than the melting transition of vortex matter, is the cause of the observed behavior.

Section: Phase Diagram Of Vortex Matter For Nb-2 Comparison With Theo...supporting

confidence: 76%

From the second magnetization peak to peak effect. A study of superconducting properties in Nb films and MgB₂bulk samples

Σταμόπουλος¹,

Speliotis²,

Niarchos³

2004

“…If the decoupling transition for the crystal with T c = 89.9 K occurs at the points defined at T 1 , the melting transition can be located on the same line (as in the case of the highly anisotropic BSCCO compound [9,[56][57][58][59]) or at the endpoint T 2 of the fishtail peak in agreement to the two-stage melting scenario [1,37]. Similar behaviour has also been observed in the case of less anisotropic YBCO [60][61][62] and BSCCO [63][64][65] crystals.…”

Section: Melting Line In a Layered Superconductorsupporting

confidence: 69%

A dc magnetization and local permeability study of the HgBa₂CuO_4+δsuperconductor

Σταμόπουλος¹,

Pissas²

2001

Local Hall probe ac permeability and global dc SQUID magnetization measurements were used to construct the phase diagram of the vortex matter for two HgBa 2 CuO 4+δ single crystals with different amounts of disorder. Emphasis was given near T c and for low magnetic fields H dc < 1 kOe. For the nearly optimally doped crystal with T c = 94.8 K, the second peak line H sp and the respective onset line H min of the magnetization loops end at (T * , H * ) = (88 K, 2 kOe) on the irreversibility line, while for the crystal of lower T c (= 89.9 K) decreases monotonically and terminates just below T c . In addition, for the crystal with T c = 89.9 K the local permeability measurements (as a function of temperature or dc magnetic field) revealed a peak in the screening current just below the onset of the diamagnetic screening. The possible sublimation character of the melting transition, the two-stage melting scenario and a Bragg to vortex glass transition are discussed. The different behaviours observed between the two crystals is attributed to the different amount of disorder.

“…This effect, although less pronounced, can be observed even at the temperature of 49.8 K. However, at this temperature E 0 /2T = 0.6625, which is about twice as large than at 39.8 K. Unfortunately, the model of Koshelev and Bulaevskii [14] does not take into account the appearance of sub-phases of vortex matter within the vortex-liquid state, i.e. of the flux-line liquid and pancake-gas, as proposed later by Blasius et al [11,12], which could explain the behaviour of B b and B r shown in figure 4.…”

Section: Resultsmentioning

confidence: 90%

“…According to Vinokur et al [10], the vortex-phase diagram of Bi 2 Sr 2 CaCu 2 O 8+δ in the B-T plane usually consists of three distinct phases: the vortex-solid phase stable over the entire temperature range below the critical temperature T c for small magnetic fields, the vortex-glass phase, stable at low temperatures, below approximately 0.5 T c and higher magnetic fields, and the remaining vortex-liquid phase, for temperatures above approximately 0.5 T c and higher magnetic fields than the vortex-solid phase. Two other possible vortex phases within the vortex-liquid phase have been proposed by Blasius et al: the flux-line liquid and pancake-gas phase [11,12].…”

Section: Introductionmentioning

confidence: 99%

Lineshape of Josephson plasma resonance in underdoped Bi₂Sr₂CaCu₂O_8+δsingle crystals

Gierłowski

Spathis

2007

We have applied theoretical results derived by Koshelev and Bulaevskii, later refined by Coffey, to fit our experimental Josephson plasma resonance data for magnetic-field sweeps of underdoped Bi2Sr2CaCu2O8+δ single crystals with a Tc of approximately 72 K, measured in the frequency range between 19.2 and 31.6 GHz. Excellent fits were obtained for reduced temperatures of up to 0.55 in the case of the vortex liquid. We have also found that, for higher temperatures, the universal scaling function f(ζ) can be used to describe the obtained results with the fitting parameters Bb and Br. These fitting parameters change remarkably below the irreversibility line, which enables a distinction to be made between the pancake-gas and flux-line liquid states. In the case of the vortex solid, our fitting procedure was also working quite well.