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Questions tagged [condensed-matter]

The study of physical properties condensed phases of matter, including solids and liquids.

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Is naturalness meaningful for non-fundamental theories?

Naturalness has been a guiding philosophy for particle physics for a long time, but a few years ago I heard a talk by Nima Arkani-Hamed where he pointed out that it seems to have failed us as it ...
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How isthe transparency of glass explained in physics?

I would like a rigorous explanation: a theory and model which describes glass as well as iron, and see why one of them is transparent; a detailed mathematical computation or a detailed reference. And,...
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The surface states and Fermi arcs in Weyl semimetals

I'm confused about surface states in Weyl semimetals. Assume that we have a single pair of Weyl points and the Fermi level turned to this points. In this https://arxiv.org/abs/1301.0330 paper the ...
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What are dark excitons and how to find them?

I am reading about excitons and I encountered a few times the term "dark exciton" but I have a hard time finding a good definition. I tried to google it but I only find scientific articles where the ...
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How can I find edge states given a bulk Hamiltonian for a topologically ordered phase?

Suppose we have a momentum space tight binding Hamiltonian $H(\vec{k})$ that describes some topologically ordered system. It could be a Chern insulator in two dimensions, or a Weyl semimetal in three ...
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How to (hypothetically) calculate $\tau$, the mean free time?

Referring to the Drude Model, I've seen a lot of excellent questions on whether $\tau$ should be thought of as the "average time between collisions" or the "average time until the next collision", and ...
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Difference in formulae

Why is there a difference of opinion in these books? Could you please provide the rigorous derivation which the author states in the second image? And why isn't the formulae same?
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Strain field and periodic boundary conditions

Let's say I have a lattice, and I impose periodic boundary conditions. I want to construct a tight-binding model on a strained lattice, and I can determine the change in the hopping parameter based on ...
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What is topological material?

Recently, topological material has been a hot topic in condensed matter physics, but I don't know what is topological material and how to distinguish topological material from band diagram. And how ...
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Question about the retarded Green's function and the analytic continuation of the Matsubara Green's function

This question is a little bit mathematical. It is about the relation between correlation function in the Matsubara frequency and the retarded correlation function in the real frequency. The following ...
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If matter comes from energy, does this mean that energy has weight?

If the matter in the Universe formed from the energy of the Big Bang, and matter has weight, does this mean that energy has weight? Moreover, as the Universe expands over time, does its overall ...
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Lattice hopping at boundary in graphene lattice with magnetic field

Let's say I have a tight binding model for graphene, where I have a two-atom basis and three nearest neighbor vectors. I've applied a homogenous magnetic field $B$ in the z-axis, and can take the ...
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What is the density of states $SiO_2$?

We build the model by the finite element method. In our model here is silicon dioxide (SiO2). To carry out calculations, it is necessary to know the density of states and the effective mass. Question:...
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Under which condition the drift velocity stops increasing proportionally with respect to the electric field

Our professor asked us a question the other day but i wasn't able to figure it out. If we increasing Electric field then drift velocity will increase, Because of this Equation: Vd = μ * E But at ...
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Symmetry arguments on the Berry connection and the polarization charge

Consider the Berry connection $$ A_n(\mathbf{k})=i \langle n(\mathbf{k})|\nabla_{\mathbf{k}}|n(\mathbf{k})\rangle $$ and the polarization charge $$ \mathbf{P}=-\frac{1}{4\pi^2} \int_\mathrm{B.Z.}\...
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Conserved charge during renormalization-group flow

Let us consider a quantum system (at zero temperature) with a continuous (anomaly-free) symmetry $G$ and there exists a corresponding conserved charge $Q$. Then we perturb this (might-be critical) ...
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Why the correlation function of 2D classical XY model is written so?

2D classical XY model $$H = -J\cos(\theta_{i}-\theta_{j})%$$ is famous for Berezinskii-Kosterlitz-Thouless phase transition. This is because of the difference of correlation function between hot and ...
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What is the analog of quantifiable magnetic field in condensed matter systems?

What is the analog of the magnetic flux density B, which is an observable and quantifiable quantity (i.e. $|B|$ can be defined in Teslas), in the anomalous QHE? For instance in the Haldane model? In ...
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Fermi gases and holes

It is a well known fact that at $T\to0$ the energy of the Fermi level for a noninteracting electron gas is given by $$E_{F}=\frac{\hbar^{2}k_{F}^{2}}{2m},\,\,k_{F}=(3\pi^{2}n)^{1/3},$$ where $k_{F}$ ...
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Help integrating potential of interaction

I'm having trouble integrating a potential that is very present in many theories regarding Condensed Matter Physics. The potential I'm trying to integrate is $$\int_{0}^{\alpha}d^3\textbf{r}\frac{1}{|\...
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In spin systems, a mean field with nonzero Chern number after Gutzwiller projection changed into trivial state?

The mean field is nontrivial because of nonzero Chern number. The gauge symmetry is Z2. Under Gutzwiller projection, I calculate the ground state degeneracy(GSD) and find that the GSD is one(trivial ...
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Physical interpretation of Bogolyubov sound with zero momentum in BEC

The point with zero momentum in the spectrum of acoustic phonon corresponds to a translation of the crystal. Is there an interpretation like this of the Bogolyubov sound in BEC? I have read that in ...
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Density of states for a tight binding model

So we have been given a dispersion relation of the form: $$ E=6-2(\cos k_xa+\cos k_ya) $$ and asked to calculate the density of states. The equation for the density of states is (eq 2.48 from here ...
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Collective modes of charge density wave

The question is about collective modes of charge density waves, i.e., amplitude and phase fluctuations $\delta,\phi$ of the order parameter $\Delta(x,t)=(\Delta_0+\delta)e^{i\phi}$. I read on p.1 of ...
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Is the DOS (density of states) wrong for degenerated case?

The density of states (DOS) is defined as $$\mathcal{N}\left(\lambda\right)=\sum_{n=1}^{M}\delta\left(\lambda-\lambda_{n}\right).$$ We can then get $$\int d\lambda\mathcal{N}\left(\lambda\right)=M,$$ ...
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Feynman diagrams: from QFT to condensed matter

I studied Feynman diagrams in quantum field theories and I'm going to study them in the context of condensed matter physics. In this post Books for Condensed Matter after Ashcroft/Mermin, two books ...
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How to interpret Berry curvature in 2-band model?

While studying the 2-band Haldane model, I realized that I am missing an intuitive picture of how Berry curvature comes into play, especially when considering an adiabatic loop. A 2-band model has 2 ...
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Landau level in particular cases

If we consider an array of identical uncoupled spinless non-interacting one-dimensional wires, as shown in Fig.(a) with a single-particle electronic dispersion E(k), which we can take to be parabolic ...
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Why is the density of a BEC so low?

I've just begun reading C. Pethick and H. Smith's textbook "Bose-Einstein condensation in dilute gases" (Cam. Uni. Press). In the Introduction, they contrast the density of atoms at the centre of a ...
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How can I simulate a ground state degenerate system numerically?

I'm using numerical method like DMRG to simulate ground state of correlated systems. But the degeneracy of the ground state has long bothered me: When degeneracy exists the ground state isn't unique. ...
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How can I compute the spin texture for a $SU(2)$ gauge model?

I am trying to determine the helicity of 4 Dirac cones in my model, and one way I want to approach it is by plotting the spin-texture. However, I am unsure of how one would calculate the spin-texture ...
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Behaviour of quantum spins

I am reading the Jordan-Wigner transformation in the book "Introduction to many-body physics" by Piers Coleman. When I read the introduction of this chapter, it is stated that: Quantum spins are ...
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What is the atomic limit?

I am attempting to grasp topological superconductivity for an assignment and in trying to understand what makes a quantum system topological have came across the following paragraph; "In the case ...
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What is the charge neutrality point in solid state physics?

I came across the term in solid state physics recently, usually in the context of a gate voltage applied to some solid in order to tune the charge carrier density. What is the definition of the ...
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What creates a negative charge in the material?

What creates a negative charge in the material? Is it an electron or a holon? Holons as I understand is a math. But what then creates an electrostatic repulsion? Is it still electron?
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Luttinger model, conform field theory and RG

I'm studying the Luttinger model and I'm having a hard time understanding its relation with conformal field theory. I'd like to know something about this. Is the Luttinger model conformal only at ...
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Wigner Seitz cell

While searching the difference between primitive cell and unit cell I have seen that "Primitive unit cells contain only one lattice point, which is made up from the lattice points at each of the ...
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What is the standard definition of quantum spontaneous symmetry breaking?

i found many answers about spontaneous symmetry breaking here but i am not sure to see what is the standard definition of SSB. i am interested in the BCS theory and i would like to know how the ...
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Momentum Space Representation of the Tight Binding Hamiltonian

I am trying to represent the tight-binding Hamiltonian \begin{equation} \hat{H}_{TB} = \sum_{\sigma} \sum_{\alpha,\beta} \sum_{\mathbf{R}_1,\mathbf{R}_2} t^{\alpha,\beta}_{\mathbf{R}_1,\mathbf{R}_2} \...
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To get short-range interaction from long-range interaction

Interactions in Condensed matter systems are almost exclusively the electromagnetic interactions which are long-range. But it often gives rise to short-range interactions in systems e.g., exchange ...
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Does pseudospin necessarily carry angular momentum?

And if so, why? I was told that anything that mathematically transforms like a spin must carry angular momentum. Is this true? One example of pseudospin is electrons in graphene on the A and B ...
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$sp^3$ Hybridization wavefunctions and probability density

I have plotted a hydrogen-like sp3-hybrid orbital probability density and it looks like this: I can plot 4 overlapping probability densities in a tetrahedral shape: So far it looks OK. But when I'm ...
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What is line broadening phenomenon in quantum many body physics?

I have been reading Anderson's paper, "Absence of diffusion in a certain Random Lattice" and found the concept of inhomogeneous broadening. I couldn't really find a satisfactory explanation with a ...
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How does Bohmian Mechanics explain superconductivity?

I'm looking for sources that discuss how Bohmian Mechanics explains superconductivity. Are there still Cooper pairs? Phonons? I saw one vague reference to vortices, but no details. This is my first ...
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Can the total energy of a solid be measured with photoemission?

Angle resolved photoemission spectroscopy (ARPES) is a really popular tool developed to study the behavior of solids by looking at the behavior of their photorlectrons. This is used to measure ...
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Where is the missing energy in the Debye-Waller effect?

X-ray/neutron scattering from crystals and liquids is well-described by the scattering theory to give the (dynamic) structure factor which is a function of momentum and energy: $$S(\mathbf{k},\omega)=...
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How to interpret overlap in Hamiltonian if it is not a degeneracy?

In Fruchart et al.'s An Introduction to Topological Insulators, the Bloch Hamiltonian for a two-band insulator is given in the general form $ H(k)= $ \begin{bmatrix} h_0+h_z & h_x-i h_y \\ ...
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Do topological transitions only occur at Dirac points?

Topological phase transitions happen when the band gap closes. It is not true that all band crossings are topological. There are Dirac (linear) band crossings, quadratic band crossings, Dirac-like ...
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second-harmonic generation and inversion centers confined to a planar defect

Let's say that we have a phase which breaks inversion symmetry. Now lets say two domains of this phase meet at a plane such that the whole system now has inversion symmetry, with the inversion ...