# Questions tagged [many-body]

Many body covers questions about systems consisting of a great number of particles and techniques used to tackle them.

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### Operators and periodic boundary conditions

Background: In Ref. 1, a system of $N$ (identical) fermions is considered. The system is enclosed in a cubic box of volume $\Omega=L^3$ and periodic boundary conditions are employed, that is (I'll ...
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### Definition of symmetry factor $p$ in Feynmans diagrams symmetry factor in Coleman's "Introduction to Many-Body Physics"

I'm trying to digest Coleman's 7.2.1 chapter about symmetry factors. Everything is clear up to point 4 where he introduces symmetry factor $p$ as the "dimension of the group of permutations under ...
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### Why are exact solutions limited to hydrogen-like atoms? [duplicate]

Why can we only find exact solutions to the Schrödinger equation for Hydrogen atoms without estimating. What is the problem with the mathematics of extending the Schrödinger equation to more ...
1 vote
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### BCS gap parameter and expansion of trace log term

I have a naive question on the solution to the Gap equation and expansion of trace log term of the effective action of BCS Hamiltonian. Solving the gap equation, one finds a famous plot of the Gap ...
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### Separable Hilbert space in quantum mechanics

When one studies quantum mechanics under a more rigorous point of view, the very first postulate states that the underlying Hilbert space $\mathscr{H}$ is separable. This means that $\mathscr{H}$ has ...
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### Energy of Jellium model

I have a stupid question and sorry for that. In jellium model, why can we disregard the kinetic energy of the ions? The approximation is that we take the density to be constant, \begin{align} \langle\...
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### Time evolution of operators in linear response theory (Kubo formula)

I am reading the following book Many-body quantum theory in condensed matter physics book by Henrik Bruus and Karsten Flensberg This book mainly focuses on time-independent Hamiltonians only. When ...
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### How do I show that $\mathrm{Tr}(\rho c_n^\dagger c_m) = \lambda_n \delta_{nm}$ implies that $\rho = \bigotimes_n \rho_n$? [closed]

Consider a many-body system with a set of $N$ fermionic modes $\{ c_n \}$ which obey the algebra $$\{ c_n,c_m^\dagger \} = \delta_{nm}, \quad \{ c_n,c_m \} = \{ c_n^\dagger,c_m^\dagger \} = 0.$$ In ...
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### Interaction Term in Tomonaga-Luttinger Model

I am studying Tomonaga-Luttinger Model from Altland and Simon's textbook called Condensed Matter Field Theory. From the derivation, I am stuck with showing that the contribution to the interaction ...
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### Principled definition of many particle wave function

In standard texts, I find no systematic and principled definition of a many-particle wave function. Perhaps I am not looking in the right standard textbooks. In my inadequate reading of the literature,...
1 vote
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### Static spin structure factor VS equal-time spin structure factor

It looks like many papers (maybe all papers containing "static spin structure factor") use the terminology, static spin structure factor, to refer to the equal-time spin structure factor ...
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### Simplifying cubic and quartic interaction operators

the probability of decay of a particle into two due to cubic in $\hat{x}$ interaction is given by $\langle f \mid \hat{x}^3\mid i\rangle$. The $\hat{x}^3$ term is written in the basis of ladder ...
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### Non-additivity of higher order terms in the intermolecular potential

The intermolecular potential energy can be written as $$u(r_{1},...,r_{N})=\sum_{i<j}^{N} u_{2} (r_{i},r_{j}) + \sum_{i<j<k}^{N} u_{3} (r_{i},r_{j},r_{k})+...$$ where the nuclear coordinates ...
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### The starting state/vacuum in Gell-Mann and Low theorem

In the proof of the Gell-Mann and Low theorem (See equation (6.38) in Fetter and Walecka for an example), we assume that at time $T \rightarrow \infty$, we start with \begin{equation} \tag{1} |\psi (-...
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### What is meant exactly by "eigenstate ensemble average"?

I am currently reading about Eigenstate Thermalization Hypothesis (ETH) and Berry's conjecture. In the paper by Srednicki on chaos and quantum thermalization, in Eq.(3.8) he calculates the average of ...
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### Fermi gas model nucleus

I was trying to figure out the solution to exercise 2.7 on Quantum theory of many particles systems by Fetter & Walecka, it asks to model the nucleus as a degenerate non-interacting Fermi gas. ...
1 vote
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### Rationale behind the Variational Monte Carlo Method

The variational method can be used to calculate the ground state wavefunction of a quantum many-body system. Suppose we have a trial wavefunction representing the ground state of the system, ...
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### Dimensions of the space of quantum states in a quantum dot

In the paper C. S. Lent and P. D. Tougaw, "A device architecture for computing with quantum dots," in Proceedings of the IEEE, vol. 85, no. 4, pp. 541-557, April 1997, doi: 10.1109/5.573 ...
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### Do all electrons in BCS ground state form Cooper pair?

I've been studying introductory superconductor theory from Solid State Physics textbook (Kittel / Ashcroft), and I found some conflicting statement from each textbook. In Kittel 8th edition, it is ...
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### How to derive the expression for the effective potential in Kohn-Sham operator in Kohn-Sham density functional theory?

In density functional theory the Kohn-Sham method provides a systematic way to approaching the correct electron density of a given system. Kohn-Sham method uses a non-interacting reference system ...
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### Does particle-hole symmetry always imply half-filling and real correlations $\langle c^\dagger_n c_{n+1} \rangle$?

Suppose we had a lattice Hamiltonian $H$ which was symmetric under the particle-hole transformation $$c_n \mapsto U^\dagger c_nU=(-1)^nc^\dagger _n$$ such that $[H,U] = 0$, where $c_n$ are Fermionic ...
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1 vote
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### Is the Rayleigh–Schrödinger perturbation theory ever useful for a many-body system?

The Rayleigh-Schrodinger perturbation theory is introduced in every textbook on quantum mechanics. It seems that it can yield accurate results for many single-particle systems. Actually, in most ...
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### Retarded and Time-Ordered Green-Function

The time-ordered and retarded-Green-functions are defined as \begin{align} G_{ \alpha \alpha^{\prime}} (t) &= - \mathrm{i} \langle T_{t} \, a_{ \alpha } ( t ) a_{ \alpha^{\prime}}^{ \dagger } ( 0 )...
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### Coherent state path integral for Hamiltonian with anomalous terms (

I know little about path integrals from Altland and Simons. When they do superconductivity with the path integral formalism they start by writing down the action with a local interaction, do the ...
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### How to calculate the second-order pertubation in a Bose gas?

I'm self-learning many body theory and right now I'm trying to solve Problem 1.3 from Quantum Theory of Many-Particle Systems by Fetter and Walecka. Problem: Given a homogeneous system of a spin-zero ... 56 views

### Natural log introduced in microstates derivative with respect to energy in equilibrium equation

In Pathria and Beale's Statistical Mechanics, 3rd ed, Chapter 1.2 (Contact between statistics and thermodynamics: physical significance of the number $Ω(N, V, E)$ ) The equation to maximize $Ω^{(0)}$ ...
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### How to get to the following result in second quantization? [closed]

I'm reading Piers Coleman's "Introduction to Many Body Theory" and I'm currently at the chapter for second quantization, at some point it gives what it calls an "heuristic derivation&...
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### Hubbard Model Hamiltonian in matrix form using basis

I am reading material on Hubbard Model (please see this link) "The limits of Hubbard model" by Grabovski, and I have difficulty deriving/calculating hamiltonian in chapter 8. eq.8.2. How ...
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### How to calculate integrals in Ideal Fermi Gas theory? [closed]

I'm having troubles solving integrals in the Ideal Fermi Gas theory. In particular the ones of the type: $$\int\frac{d\vec{k}}{(2π)^3}θ(k_F − k)( \vec{k} \cdot \vec{q})^n$$ but I actually don't ... 37 views

### Nonvanishing expectation value lesser Green's function

Consider bosonic field operators in the Heisenberg picture: \begin{align} \Psi(x)=\int \frac{d^{3}p}{(2\pi)^{3}}e^{-ip\cdot x}a_{\bf{p}}\\ \Psi^{\dagger}(x)=\int \frac{d^{3}p}{(2\pi)^{3}}e^{+ip\cdot x}...
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### Second-Order Perturbation in electron gas

I was trying to figure out the solution to exercise 1.4 on Quantum theory of many particles systems by Fetter & Walecka and I read through this question and its answer. But a point made in both ...
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### Path integral on many-body quantum mechanics

Suppose $\mathscr{H}$ is a Hilbert space describing a one-particle quantum system and $\mathcal{F}(\mathscr{H})$ is its associated Fock space, which is used to describe a many-body quantum system. Let ...
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### How to use Density Functional Theory to get the correct energy spectrum

I'm having some trouble understanding how DFT can be used to obtain results for electronic structure calculations. We can assume that we're studying an $N$ electron atom with a fixed nucleus and want ...
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### Frequency Integration of Green's Function

If one has a Green's Function that has been projected into the Helcity basis (from spin) of the from \begin{equation} G(\mathbf{k},\omega)=\sum_s\frac{1}{\omega-\epsilon_{\mathbf{k},s}+\mu+isgn(\omega)...
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### Why can we distinguish two electrons in different experiments?

Electrons are indistinguishable particles, however, when I set up two independent experiments (at two positions), I can talk about "the electrons in Experiment x". What's going on here? I ...
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### Fock Space and Coherent state

Can a coherent photon state also belong to the Fock space? If yes, under what conditions? For example I read that \exp\bigg\{-\frac{1}{2}\sum_i|\alpha_i|^2\bigg\}\exp\bigg\{-\sum_i\alpha_ia_i^{\...
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### Representation of one-body operator in field theory

Section 2.1 Introduction to second quantization, Page No 47 of Condensed Matter Field Theory reads Representation of operator (one-body) Single-particle or one-body operator $\mathcal{O}_1$ acting in ...
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