Simple Ginzburg-Landau Theory for Vortex Lattice Transformations in Four-fold Symmetric Superconductors

여준현,Myoung Kwan Ko 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.1

We use a simple phenomenological model introduced by Yeo and Moore to study the vortex lattice structural transformation in the presence of fourfold symmetric coupling to an underlying lattice as in borocarbide superconductors. We focus on the aspects of the model that were not previously exploited, including the calculations of the transition line between the rhombic and the square vortex lattice phases and of the critical exponent describing the transition. We also nd that, when the model parameter is interpreted appropriately, the model is consistent with the reentrant transition behavior of the vortex lattice observed in recent experiments.

Two-Loop Renormalization Group Analysis of the Vortex Liquid-to-Crystal Transition in Type-II Superconductors

여준현 한국물리학회 2002 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.41 No.1

We present a functional renormalization group (RG) study of the Ginzburg-Landau (GL) model for a type-II supercondutor in a magnetic field. We calculate the {\it two-loop} RG flow equations for the coupling functions of the GL theory both for a pure system and for s system with quenched random impurities. We confirm the validity of the conjectured form for the coupling functions used to obtain the stable fixed point for the disordered model in the earlier one-loop functional RG analysis [M.\ A.\ Moore and T.\ J.\ Newman, Phys.\ Rev.\ Lett.\ {\bf 75}, 533 (1995)]. We also show explicitly that the one-loop result claiming that the fixed point for a disordered superconductor in $d$ dimensions with $N/2$ order parameters is identical to that of the disordered $O(N)$ model in $d-2$ dimensions remains valid to the two-loop order.

Towards an effective theory for vortices in layered superconductors

Yeo, Joonhyun 建國大學校基礎科學硏究所 2001 理學論集 Vol.26 No.-

We present a possible step towards constructing an effective theory for vortices in layered superconductors. Starting from the Lawrence-Doniach model, we transform the inter-layer coupling term appropriately to construct an effective Hamiltonian. In the present scheme, all the thermodynamic quantities in a layered system are expressed in terms of the corresponding quantities describing two-dimensional vortex liquids.

2차원 vortex 액체를 기술하는 비섭동적 방정식의 수치적 연산 방법

박현준,여준현 한국물리학회 2014 새물리 Vol.64 No.4

The parquet approximation is a nonperturbative and analytic method for studying the phase transitions in the vortex system of type-II superconductors. Using this approximation, one can calculate the structure factor of vortex liquids from the vertex function arising in the Ginzberg-Landau free energy. Unlike numerical Monte Carlo simulations, there is no finite size effect because the calculation is done in an infinite area. The parquet approximation gives the so-called parquet equations in the form of complicated self-consistent integral equations, and one has to resort to numerical methods to solve those equations. Although the phase transitions in the vortex system has been studied before by using the parquet equations, numerical results have not yet been presented in detail. In this paper, we explicitly present numerical methods for obtaining solutions to the parquet equations. We develop an efficient method using the symmetry in the problem and apply it to the case where non-parquet contributions from the irreducible diagrams are included, which is not studied in the conventional parquet approximation. 제 II형 초전도체 박막의 혼합 (mixed) 상태에서 존재하는 vortex들이 만드는 물리계의 상전이를 연구하는 방법으로 비섭동적이며 해석적인 방법인 parquet 방정식을 사용하는 방법이 있다. 이는 Ginzburg-Landau 자유에너지에서 주어지는 vertex 함수를 이용하여 vortex 액체의 구조인자 (structure factor)를 계산하는 방법으로 몬테카를로 시뮬레이션 (MonteCarlo Simulation)과 같은 수치계산과는 달리 무한 영역에서 계산하기 때문에 finite size effect가 없다. 그러나 parquet 방정식은 결합된 형태의 복잡한 self-consistent 방정식이기 때문에 해석적인 방법임에도 불구하고 그 해를 구하기 위해서 수치적 계산 방법을 이용해야 한다. 이 방정식을 이용한 vortex계의 상전이 연구는 이미 여러 논문을 통해 다루어졌다. 그러나 parquet 방정식의 수치적 해법에 대해서는 구체적으로 소개된 적이 없다. 이 논문에서는 parquet방정식의 해를 수치적으로 구하는 구체적인 방법을 제시하고, 대칭성을 이용하는, 보다 효율적인 계산 방법을 제시한다. 또한 보통의 parquet 방정식에서 포함하지 않는 기약 diagram이 포함된 non-parquet 기여가 존재하는 경우에 대한 구체적인 계산 방법을 다룬다.

Vortex 액체를 기술하는 Parquet 방정식의 첫 번째 Non-parquet 기여에 대한 수치적 계산

박현준,여준현 건국대학교 기초과학연구소 2006 理學論集 Vol.31 No.-

Zn-Co 합금도금의 부식거동에 대한 코발트의 효과

김정구,여준현,이재융 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.5

Recently Zn-Co alloy coating has been studied because minute additions of cobalt to zinc decrease its rate of corrosion. The corrosion behavior of electrodeposited Zn-Co alloy coatings with various contents of Co was compared with pure Zn coating using electrochemical test and electrochemical impedance spectroscopy (EIS) measurement in 3.5 wt% NaCl solution and surface analysis. In immersion test, the corrosion potential of all Zn-Co coatings was higher than that of pure Zn coating. Corrosion current density for the 0.9 wt%Co and 5.0 wt%Co was lower than that for pure Zn coating. Dezincification which occurred in the corrosion process made a Co-rich barrier in the coating layer and only 0.9 wt%Co and 5.0 wt%Co had an effective Co-rich barrier. The 58.0 wt%Co exhibited the highest current density owing to the localized corrosion at large cracks. The 0.9 wt%Co and 5.0 wt%Co, with a more active corrosion potential, were galvanically corroded at cracks in the coating to protect the exposed substrate cathodically. Whereas on the 58.0 wt%Co, anodic dissolution and galvanic corrosion of the substrate were concentrated at coating defects by a large cathode(58.0 wt%Co coating) and anode(substrate) surface area ratio. EIS and EPMA results confirmed that cobalt enrichment, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate.

Integral Equation Approach to Correlations in a Two-Dimensional Two-Component Hard-Core Plasma

Minwoong Joe,여준현 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.4

The hypernetted chain (HNC) and the Percus-Yevick (PY) approximations are combined and applied to a two-dimensional two-component hard-core plasma system, which consists of symmet- rically charged hard discs interacting through a Coulomb potential. The HNC approximation is known to be suitable for treating long-range Coulomb interactions. We show how to incorporate the short-range hard-core repulsion into the HNC integral equations by using the PY approximation. The pair correlation functions between same and dierent charges are calculated for various values of the temperature and the density.

Unconventional Entropy Production in the Presence of Momentum-dependent Forces

Chulan Kwon,여준현,Hyun Keun Lee,Hyunggyu Park 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.5

We investigate the unconventional nature of entropy production (EP) in nonequilibrium systems with odd-parity variables that change signs under time reversal. We consider the Brownian motion of a particle in contact with a heat reservoir, where the particle’s momentum is an odd-parity variable. In the presence of an external momentum-dependent force, the EP transferred to the environment is found to be not equivalent to the usual reservoir entropy change due to heat transfer. An additional unconventional contribution to the EP, which is crucial for maintaining the non-negativity of the (average) total EP enforced by the second law of thermodynamics, appears. A few examples are considered to elucidate the novel nature of the EP. We also discuss detailed balance conditions with a momentum-dependent force.

Finite-Size Effects in Monte Carlo Simulations of the Gaussian Disorder Model

Sunil Kim,여준현,임찬,김도석 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.60 No.11

We study theoretical issues related to the use of finite size systems in Monte Carlo simulations of the Gaussian disorder model (GDM). The GDM is one of the most widely used models to describe the charge transport in random organic materials. In the GDM, the energy of charge carriers is well known to approach an equilibrium energy in the long-time limit. We find that at low temperatures, the equilibrium energy shows a strong dependence on the system size. Using improved numerical methods, we study system sizes much bigger than those used in earlier works. We show that, below a certain temperature, a system size much larger than those used in conventional studies must be used to correctly describe the temperature dependence of the equilibrium energy.

Role of Non-Parquet Diagrams in the Parquet Graph Resummation Method for Two-Dimensional Vortex Liquids

Hyunjoon Park,여준현 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.4

We study the role of the non-parquet diagrams in a nonperturbative approach called the parquet graph resummation method for studying correlations in a two-dimensional vortex liquid. As an analytic approach, the parquet graph resummation method captures the growing crystalline order in the vortex liquid as the temperature decreases. For its systematic improvement, it is important to understand the effects of the non-parquet diagrams, which are neglected in a usual approximation. The contribution of the non-parquet diagrams can be summarized into the non-parquet vertex function that appears in the lowest Landau level formalism of the Ginzburg-Landau theory of a superconductor in a magnetic field. We find that, as the temperature decreases, the non-parquet vertex develops minima at positions of the reciprocal lattice vectors of a triangular lattice where peaks grow in the structure factor of a vortex liquid. By solving the parquet equations for various forms of the non-parquet vertex, we analyze the effects of the minima in the non-parquet vertex on the positions and the sizes of the peaks in the structure factor. We study the role of the non-parquet diagrams in a nonperturbative approach called the parquet graph resummation method for studying correlations in a two-dimensional vortex liquid. As an analytic approach, the parquet graph resummation method captures the growing crystalline order in the vortex liquid as the temperature decreases. For its systematic improvement, it is important to understand the effects of the non-parquet diagrams, which are neglected in a usual approximation. The contribution of the non-parquet diagrams can be summarized into the non-parquet vertex function that appears in the lowest Landau level formalism of the Ginzburg-Landau theory of a superconductor in a magnetic field. We find that, as the temperature decreases, the non-parquet vertex develops minima at positions of the reciprocal lattice vectors of a triangular lattice where peaks grow in the structure factor of a vortex liquid. By solving the parquet equations for various forms of the non-parquet vertex, we analyze the effects of the minima in the non-parquet vertex on the positions and the sizes of the peaks in the structure factor.