Browsing by Author "Li, DP"

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Lowest Landau level approximation in strongly type-II superconductors
(1999-10-01)
Li, DP
;
Rosenstein, B
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
Higher than the lowest Landau level, (LLL) contributions to magnetization and specific heat of superconductors are calculated using the Ginzburg-Landau equation approach. Corrections to the excitation spectrum around the solution of these equations (treated perturbatively) are found. Due to symmetries of the problem leading to numerous cancellations the range of validity of the LLL approximation in the mean held is much wider then a naive range and extends ail the way down to H=H-c2(T)/13. Moreover, the contribution of higher Landau levels (HLL) is significantly smaller compared to LLL than expected naively. Like the LLL part, the lattice excitation spectrum at small quasimomenta is softer than that of the usual acoustic phonons. This enhances the effect of fluctuations. The mean-held calculation extends to third order, while the fluctuation contribution due to HLL is to one leap. This complements the earlier calculation of the LLL part to two-loop order, [S0163-1829(99)03834-5].
Melting of the vortex lattice in high-T-c superconductors
(2002-06-01)
Li, DP
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
The precise measurements of vortex melting point towards a need to develop a quantitative theoretical description of thermal fluctuations in vortex matter. To tackle the difficult problem of melting, the description of both the solid and the liquid phase should reach the precision level well below 1%. Such a theory in the framework of the Ginzburg-Landau approach is presented. The melting line location is determined, and magnetization and specific-heat jumps along it are calculated. We find that the magnetization in the liquid is larger than that in the solid by 1.8% regardless of the melting temperature, while the specific-heat jump is about 6% and slowly decreases with temperature. The magnetization curves agree with experimental results on Y-Ba-Cu-O and Monte Carlo simulations.
Optimized perturbation theory in the vortex liquid of type-II superconductors
(2002-01-01)
Li, DP
;
Rosenstein, B
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
We develop an optimized perturbation theory for the Ginzburg-Landau description of thermal fluctuations effects in the vortex liquids. Unlike the high temperature expansion which is asymptotic, the optimized expansion is convergent. Radius of convergence on the lowest Landau level is a(T)= -3 in two dimensions (2D) and a(r)= -5 in three dimensions (3D). It allows a systematic calculation of magnetization and specific heat contributions due to thermal fluctuations of vortices in strongly type-II superconductors to a very high precision. The results are in good agreement with existing Monte Carlo simulations and experiments. Limitations of various nonperturbative and phenomenological approaches are noted. In particular we show that there is no exact intersection point of the magnetization curves both in 2D and 3D.
Precision calculation of magnetization and specific heat of vortex liquids and solids in type-II superconductors
(2001-04-16)
Li, DP
;
Rosenstein, B
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
A new systematic calculation of magnetization and specific heat contributions of vortex liquids and solids is presented. We develop an optimized perturbation theory for the Ginzburg-Landau description of thermal fluctuations effects in the vertex liquids. The expansion is convergent in contrast to the conventional high temperature expansion which is asymptotic. In the solid phase we calculate the first two orders which are already quite accurate. The results are in good agreement with existing Monte Carlo simulations and experiments. Limitations of various nonperturbative and phenomenological approaches are noted. In particular, we show that thew is no exact intersection point of the magnetization curves.
Quantitative theory of thermal fluctuations and disorder in the vortex matter
(2006-01-01)
Li, DP
;
Rosenstein, B
;
Lin, P
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電子物理學系
;
Department of Electrophysics
A metastable supercooled homogeneous vortex liquid state exists down to zero fluctuation temperature in systems of mutually repelling objects. The zero-temperature liquid state therefore serves as a (pseudo) 'fixed point' controlling the properties of vortex liquid below and even around the melting point. Based on this picture, a quantitative theory of vortex melting and glass transition in Type II superconductors in the framework of Ginzburg-Landau approach is presented. The melting line location is determined and magnetization and specific heat jumps are calculated. The point-like disorder shifts the line downwards and joins the order-disorder transition line. On the other hand, the disorder induces irreversible effects via replica symmetry breaking. The irreversibility line can be calculated within the Gaussian variational method. Therefore, the generic phase diagram contains four phases divided by the irreversibility line and melting line: liquid, solid, vortex glass and Bragg glass. We compare various experimental results with the theoretical formula.
Structure and orientation of the moving vortex lattice in clean type-II superconductors
(2004-12-01)
Li, DP
;
Malkin, AM
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Rosenstein, B
;
交大名義發表
;
電子物理學系
The dynamics of the moving vortex lattice is considered in the framework of the time-dependent Ginzburg-Landau equation neglecting the effects of pinning. At high flux velocities the pinning dominated dynamics is expected to crossover into the interactions dominated dynamics for very clean materials recently studied experimentally. The stationary lattice structure and orientation depend on the flux flow velocity. For relatively velocities VV-c. The two orientations can be desribed as motion "in channels" and motion of "lines of vortices perpendicular to the direction of motion." Although we start from the lowest Landau level approximation, corrections to conductivity and the vortex lattice energy dissipation from higher Landau levels are systematically calculated and compared to a recent experiment.
Supercooled vortex liquid and quantitative theory of melting of the flux-line lattice in type-II superconductors
(2004-10-01)
Li, DP
;
Rosenstein, B
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
A metastable homogeneous state exists down to zero temperature in systems of repelling vortices. A zero- "fluctuation-temperature" liquid state therefore serves as a (pseudo) "fixed point" controlling the properties of vortex liquid below and even around melting point. Based on this picture for the vortex phase we apply the Borel-Pade resummation technique to develop a quantitative theory of the vortex liquid for the lowest-Landau-level Ginzburg-Landau model in type-II superconductors. While on the solid phase, there exists a superheat solid phase which ends at the spinodal line. The picture for the vortex phase is supported by an exactly solvable large N Ginzburg-Landau model in a magnetic field and has been recently confirmed by the experiments. The applicability of the lowest-Landau-level model is discussed and corrections due to higher levels are calculated. The melting line is located based on the quantitative theory for the description of the vortex solid and the vortex liquid. Magnetization, entropy, and specific heat jumps along the melting line are calculated. The theoretical results explain quantitatively very well the experimental data on the high-T-c cuprates YBa2Cu3O7, DyBCO, low-T-c material (K, Ba) BiO3, and also Monte Carlo simulation results.
Supercooled vortex liquid and superheated vortex solid in type II superconductors
(2005-06-01)
Chen, L
;
Li, DP
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
metastable homogeneous state exists down to zero temperature in systems of repelling objects. Zero "fluctuation temperature" liquid state therefore serves as a (pseudo) "fixed point" controlling the properties of vortex liquid below and even around melting point. There exists Madelung constant for the liquid in the limit of zero temperature which is higher than that of the solid by an amount approximately equal to the latent heat of melting. On the other hand, the superheated solid exists until the spinodal line. This picture is supported by an exactly solvable large N Ginzburg - Landau model in magnetic field and it is confirmed by the recent experiment.
Theory of the vortex matter transformations in high-T-c superconductor YBCO
(2003-04-25)
Li, DP
;
Rosenstein, B
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
Flux line lattice in type II superconductors undergoes a transition into a "disordered" phase such as vortex liquid or vortex glass, due to thermal fluctuations and random quenched disorder. We quantitatively describe the competition between the thermal fluctuations and the disorder using the Ginzburg-Landau approach. The following T-H phase diagram of YBCO emerges. There are just two distinct thermodynamical phases, the homogeneous and the crystalline one, separated by a single first order transition line. The line, however, makes a wiggle near the experimentally claimed critical point at 12 T. The "critical point" is reinterpreted as a (noncritical) Kauzmann point in which the latent heat vanishes and the line is parallel to the T axis. The magnetization, the entropy, and the specific heat discontinuities at melting compare well with experiments.
Thermal fluctuation correction to magnetization and specific heat of vortex solids in type-II superconductors
(2002-01-01)
Li, DP
;
Rosenstein, B
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
A systematic calculation of magnetization and specific beat contributions due to fluctuations of vortex lattice in strongly type-II superconductors to precision of 1% is presented. We complete the calculation of the two loop low temperature perturbation theory by including the umklapp processes. Then the Gaussian variational method is adapted to calculation of thermodynamic characteristics of the two-dimensional and the three-dimensional vortex solids in high magnetic field. Based on it as a starting point for a perturbation theory we calculate the leading correction providing simultaneously an estimate of precision. The results are compared to existing nonperturbative approaches.
Thermal fluctuations and disorder effects in vortex lattices
(1999-10-01)
Li, DP
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
We calculate using loop expansion the effect of fluctuations on the structure function and magnetization of the vortex lattice and compare it with existing Monte Carlo results. In addition to renormalization of the height of the Bragg peaks of the structure function, there appear characteristic saddle shape "halos" around the peaks. The effect of disorder on magnetization is also calculated. All the infrared divergencies related to soft shear cancel. [S0163-1829(99)11837-X].