ORAL SESSION 2B2: Vortex Phase Diagram

Tuesday, Feb. 22, 10:15 a.m. – 12:30 p.m., General Assembly Hall A (GRB)

Chairs: G. Blatter (ETH Zürich), V. Vinokur (Argonne Nat'l. Lab)

2B2.1 The thermodynamic phase diagram of vortex matter in YBa₂Cu₃O_7-d

C. Marcenat 1, F. Bouquet 1, R. Calemczuk 1, W.K. Kwok 2, U. Welp 2, G.W. Crabtree 2, N.E. Phillips 3, and R.A. Fisher 3. 1 CEA/Grenoble/DRFMC/LCP, 38054 Grenoble, France. 2 Center for Superconductivity, Argonne National Laboratory, Argonne, IL 60439, USA. 3 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

Presenting Author: C. Marcenat

We report measurements of both the specific heat (C) and the magnetization (M) of a naturally untwinned YBC0 single crystal in magnetic field (H) up to 26 Tesla. The specific heat data determine the thermodynamic nature of the melting and show its evolution with increasing H within a low-H region (0.1< H<10.5 Tesla) in which it is first order, with a discontinuity in C and a hysteretic latent heat, through a multicritical point, and into a high-H region (10.5 < H < 26 Tesla) in which it is second order, with only a discontinuity in C. The qualitative change in melting in that latter region seems to reflect not a change in the nature of the solid but rather that of the (reversible) liquid. The M data show the relation of irreversibility to melting and allow to distinguish between the thermodynamic features at melting to disorder-dependent properties of the solid.

2B2.2 Effect of Defects on the Critical Points in YBa₂Cu₃O_7-d

Wai-Kwong Kwok 1, Robert J. Olsson 1, Goran Karapetrov 1, Lisa M. Paulius 2, William G. Moulton 3, David J. Hofman 4, and George W. Crabtree 1. 1 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA. 2 Department of Physics, Western Michigan University,Kalamazoo, MI 49008, USA. 3 Department of Physics, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306, USA. 4 Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA.

Presenting Author: W.-K. Kwok

The upper and lower critical points in untwinned YBa₂Cu₃O_7-d single crystals with dilute columnar defects induced by 1.4GeV 20₈Pb⁵⁶⁺ ion irradiation are investigated. At low fields, we find a distinctive Bose glass transition line in the H-T phase diagram which converges into a first order vortex melting line at the lower critical point. With increasing columnar defect density, the lower critical point shifts to higher fields. Our results suggest the existence of a new transition or cross-over line in the vortex solid state which separates a disordered Bose glass state at low fields from an ordered vortex lattice state at high fields. In addition, we find that columnar defects raise the upper critical point. Our results demonstrate that vortex line meandering due to point defects can be suppressed by columnar defects and imply that the transition near the upper critical point is a vortex entanglement transition.

2B2.3 New equilibrium phase diagram of YBa₂Cu₃O_y under high magnetic fields

Terukazu Nishizaki 1,2, Kenji Shibata 1, Takahiko Sasaki 1,2, and Norio Kobayashi 1,2. 1 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. 2 CREST, Japan Science and Technology Corporation, Kawaguchi, 332-0012, Japan.

Presenting Author: T. Nishizaki

We have measured magnetization and resistivity in untwinned YBa₂Cu₃O_y (YBCO) single crystals under high magnetic fields up to 30T. We find that the first-order vortex lattice melting line T_m(H) and the second-order vortex glass transition line T_g(H) terminate at the critical point Hcp and the field-driven disordering transition line separates the vortex solid phase into the Bragg glass and the vortex glass phases [1]. The value of H_cp strongly depends on the oxygen content and the vortex lattice melting transition is observed up to 30 T for fully oxidized YBCO (y ~ 7, T_c = 87.5 K) [2]. For optimally doped YBCO (T_c ~ 93 K), on the other hand, T_g(H) decreases with increasing temperature and approaches to the vortex lattice melting line well below the critical point of T_m(H), indicating the existence of the new vortex state such as a vortex slush regime. Thermodynamic properties are also examined above and below the terminal point of T_g(H). We find that the entropy change at the first-order melting transition becomes considerably small above the terminal field. The novel vortex phase diagram is discussed.

[1] T. Nishizaki et al., Phys. Rev. B 58, 11169 (1998).

2B2.4 Dual symmetry breaking and vortex-loop unbinding in 3D type-II superconductors

Asle Sudboe, Joakim Hove, and Anh Kiet Nguyen, Department of Physics, Norwegian University of Science and Technology, N-7491 Trondheim, Norway

Presenting Author: A. Sudboe

Transverse phase-fluctuations of the Ginzburg-Landau order parameter of type-II superconductors induces topological defects in the form of closed vortex-loops into the superconducting condensate.

This topological sector of the Ginzburg-Landau field theory may itself be written as a field theory of a local complex matter field coupled to a gauge-field mediating a long-ranged interaction between the toplogical defects; this constitutes a dual description of the Ginzburg-Landau theory (GLT), which wedenote the DGLT.

Remarkably, in three dimensions only, the dual theory exhibits a tantalizing isomorphism to the original GLT: The GLT and DGLT may be viewed as realizations of one and the same theory, the difference being the charge of the original and dual condensate.

These connections, and their consequences, will be explained. In particular, the scaling dimensions of the GL order parameter and its dual counterpart are obtained directly. These results, when viewed in conjunction with the above isomorphism, show unequivocally that neutral and charged condensates have critical behaviors belonging to different universality classes.

Furthermore, the anomalous scaling dimensions of the gauge-fields in the GLT and the DGLT are found, and their experimental consequences discussed.

Finally, the dual description provides a local order parameter for a new U(1)-symmetry breaking thermodynamic phase-transition taking place inside the molten phase of the Abrikosov vortex lattice of type-II superconductors in magnetic fields. This transition would not be possible to detect using the original GL order parameter, or any local function of it.

2B2.5 Critical anisotropy in Josephson-vortex systems induced by magnetic fields along ab plane of high-T_c superconductors

X. Hu and M. Tachiki, National Research Institute for Metals, Tsukuba 305-0047, Japan

Presenting Author: X. Hu

Large scale Monte Carlo simulations are performed for the vortex states of extremely type-II, layered superconductors in an in-plane magnetic field. The model hamiltonian is the 3D anisotropic, frustrated XY model, in which fluctuations in the magnetic flux density and in the amplitude of superconducting order parameter are neglected. For anisotropy constant less than a critical value, including the isotropic case, a first-order phase transition from the flux-line liquid to superconducting flux-line lattice is observed [1]; For anisotropy larger than the critical value, the normal to superconducting phase transition is second order [2]. The critical anisotropy is estimated as 7.0 for the flux filling factor f=1/25 [1]. As unveiled by the present simulation, many in-plane overhangs and vortex loops are thermally excited for highly anisotropic systems, and smear out the sharp onset of the flux-line entanglement, which is observed both in less anisotropic systems in in-plane fields and in systems in c-axis magnetic fields regardless the anisotropy.

[1] X. Hu and M. Tachiki, Bulletin of APS, vol. 44, No.1, 235 (1999).

[2] X. Hu and M. Tachiki, Phys. Rev. Lett. vol. 80, 4044 (1998).

2B2.6 The Universal Scaling in Magnetic Phase Diagram of HTSC

Kohji Kishio, Department of Applied Chemistry and Department of Superconductivity, School of Engineering, University of Tokyo, Bunkyo-ku, Tokyo, Japan

Presenting Author: K. Kishio

The magnetic phase diagram of HTSC compounds is substantially more complex than that in conventional superconductors. However, it is very rich in novel physics and extremely important in developing practical superconductors for technical applications.

Over the past years, we have been extensively and systematically studying the vortex behavior of various HTSC single crystals both by magnetic and transport measurements. When g ²B vs. T/T_c , for these clean materials are plotted, it is found that the phase boundaries such as first order phase transition, dimensional crossover, and irreversibility lines are all universally scaled to result in the generic magnetic phase diagram of HTSC.

We have found that the most important parameter to govern the magnetic phase diagram is the electromagnetic anisotorpy factor g , as well as the in-plane penetration depth l , in the supercon-ducting state both of which show very good correlation with the normal state conductivity.

We conclude that the interlayer coupling along the c-axis both in the superconducting and normal states is the most critical factor to determine the magnetic behavior of HTSC compounds.

2B2.7 Vortex Lattice Transitions in Tetragonal Crystals

V.G. Kogan, Ames Laboratory-DOE and Department of Physics & Astronomy, Iowa State University, Ames, IA 50011

Presenting Author: V.G. Kogan

Nonlocal corrections to London equations for superconductors with anisotropic Fermi surface are employed to describe effects of crystal symmetry on vortex lattices (VL) of high - k materials at low temperatures. The model provides a satisfactory description of the VL structures observed in neutron scattering, decoration, and STM experiments in borocarbides, VL orientations, field and mean-free path dependencies, and structural phase transitions.

Recent results on the nature of 45-degree reorientation in tetragonal materials (with the field along c-axis), which has been considered as the 1st order phase transition, will be discussed.