The vortex depinning transition in untwinned YBaCuO using complex impedance measurements

HAL Id: hal-00000937

https://hal.archives-ouvertes.fr/hal-00000937

Preprint submitted on 10 Dec 2003

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The vortex depinning transition in untwinned YBaCuO using complex impedance measurements

Alain Pautrat, Christophe Goupil, Charles Simon, Bart Andrewjewski, A.I.

Rykov, S. Tajima

To cite this version:

Alain Pautrat, Christophe Goupil, Charles Simon, Bart Andrewjewski, A.I. Rykov, et al.. The vortex

depinning transition in untwinned YBaCuO using complex impedance measurements. 2003. �hal-

00000937�

ccsd-00000937 (version 1) : 10 Dec 2003

The vortex depinning transition in untwinned YBaCuO using complex impedance

measurements.

A. Pautrat, C. Goupil, Ch. Simon, ^a B. Andrewjewski, ^b A.I. Rykov, S. Tajima. ^c

a CRISMAT, ISMRA, UMR 6508 du CNRS, Caen, France.

b Institute of molecular physics, Poznan, Poland.

c ISTEC, Tokyo, Japan.

Abstract

We present surface impedance measurement of the vortex linear response in a large untwinned YBCO crystal. The depinning spectra obtained over a broad frequency range (100 Hz- 30 MHz) are those of a surface pinned vortex lattice with a free flux flow resistivity (two modes response). The critical current in the ”Campbell” like regime and the flux flow resistivity in the dissipative regime are extracted. Those two parameters are affected by the first order transition, showing that this transition may be related to the electronic state of vortices.

The linear ac response of a pinned vortex lattice has been observed in the 60’s in conventional type II superconducting materials [1]. At low frequencies, say f . 1 KHz, a small ac field has an apparent penetration characterized by a static regime without any loss (the Campbell regime). It has been recognized as a direct consequence of the vortex pinning, and disagrees with first interpreta- tions based on thermally activated depinning [2]. Note that this linear regime is not the ohmic regime of a vortex lattice free from any pinning (the so called liquid phase). At the same time, the high frequency response (f & 1-10 MHz) is that of a medium with a free resistivity (skin effect). Those two regimes are separated by the depinning frequency, and the shape of the whole spectrum allows to discriminate between different types of pinned vortex states. One of the main interest of this method is that vortex states can be studied even in the presence of a large critical current. The principle of the experiment is to measure the ac penetration length, due to the vortex response, in a small coil directly glued around the sample. The detailed experimental procedure can be found in [3]. A typical depinning spectrum is shown in the figure 1.

It follows closely, over the whole frequency range, the two modes model of

Preprint submitted to Elsevier Science 10 December 2003

0 10 20 30 40 50 60 70

1 100 10 ⁴ 10 ⁶ 10 ⁸ 10 ¹⁰

L s = 48.5 ± 0.2 µm

λ ’

λ ’’

ρ ff = 6.4 ± 0.2 µΩ.cm

f (Hz)

f ^p

Fig. 1. Two modes response of the pinned vortex lattice in untwinned YBaCuO ( T = 88 . 6 K < T _m , B//a-axis=6 T).

surface vortex pinning developed in [4,5]. At the same time, it was impossible to perform acceptable fits using bulk pinning models. We conclude that the pinning in this YBaCuO sample is due to the surface roughness. We have also performed a precise study of the low frequency behavior close to the temper- ature of the first order transition. Within experimental accuracy, we do not find any evidence of a thermally assisted depinning regime. As the depinning frequency is directly measured by this method, we can identify the pure skin effect regime and measure the flux flow resistivity. A step in the flux flow resistivity is clearly identified at the temperature of the first order transition.

At the same temperature, the low frequency penetration length diverges up to reach the full penetration (no pinning anymore). The first order transition affects both the critical current and the flux flow resistivity. It could suggest that this transition is linked to the pairing symmetry [6].

References

[1] P. Alais, Y. Simon, Phys. Rev. 158, 426 (1967).

[2] A.M. Campbell, J. Phys. C 2, 1492 (1969).

[3] A. Pautrat et al, Phys Rev B 63, 054503 (2001) and Eur. Phys. J. B 33,279 (2003).

[4] E.B. Sonin, A.K. Tangantsev, K.B. Traito, Phys. Rev. B 46, 5830 (1992).

[5] N. L¨ u tke-Entrup et al, Phys Rev Lett 79, 2538 (1997).

[6] G. E. Volovik, JETP Lett 65, 491 (1997).

2