Notes on Seneca's Selected Essays
On the Happy Life Seneca defines the happy life as possessing a soul that is free, upright, and steadfast — one liberated from all fear and desire (p. 63). For him, happiness is not found in outwa...
Sangjun Sim
About
I write technical notes while learning condensed matter physics, many-body theory, magnetism, symmetry, topology, and programming tools. I am enthusiastic about numerical approaches to understand intriguing phenomena in solids. This blog is a place to keep ideas precise enough to revisit. I also hope that these notes can be helpful to others who are learning similar topics.
Kitaev term from symmetry
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A short note on pair-correlation function
A short note on pair-correlation function The pair-correlation function is the concept in the many-body system that can be applied to understand the physical origin of the exchange-correlation ene...
A short note on pseudopotential
A short note on pseudopotential Pseudopotentials are artificial potentials created in atomic configurations that smooth out potentials in the core region. This method assumes that the core state i...
A short note on Janak theorem
A short note on Janak theorem The total energy derived by the Kohn-Sham (KS) equation, $E[n]$ inherently does not have any physical meaning. In the Hartree-Fock approximation, the energy required ...
A short note on the Bloch theorem
Bloch theorem The Bloch theorem assumes the translational invariance of the potential $V(\mathbf{r})$. Here, the translational symmetry of $V(\mathbf{r})$ means that the potential at $\mathbf{r}$ ...
Spin Stiffness
Spin stiffness Definition of spin stiffness The spin stiffness is a constant that represents the change in the ground state energy of a spin system as a result of introducing a slow in-plane...
Berezinskii-Kosterlitz-Thouless transition (2)
Beyond spin waves: 2D $XY$ model The phase transition associated with the vortices is called the Berezinskii–Kosterlitz–Thouless (BKT) transition [1, 2]. This transition occurs in a 2D model (no ...
Refactoring Notes
What is Refactoring 리팩토링은 소프트웨어가 겉으로 드러내는 방식은 유지하면서 코드 내부 구조를 개선시키는 작업을 의미한다. 리팩토링과 단순히 재구성하는 것의 차이는 코드를 정리할 때 “특정한 방식”을 사용하는가에 있다. When to Refactoring 돈 로버츠는 비슷한 일을 “세 번째로 하게 되면” 리팩토링을 해야 한다(3의...
Berezinskii-Kosterlitz-Thouless transition (1)
This document briefly discusses the example of a topological phase transition: the Berezinskii-Kosterlitz-Thouless (BKT) transition. Berezinskii-Kosterlitz-Thouless transition Mermin-Wagner theor...