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

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Do all the electrons form Cooper pairs at absolute zero?

At $T=0K$, do all the electrons form Cooper pairs, or just the electrons near the Fermi Surface do?
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130 views

Ground state Phase Diagram of Bose-Hubbard Model

The Hamiltonian of Bose-Hubbard model reads as $$H=-J\sum\limits_{<i,j>}b_i^{\dagger}b_j+h.c.+\frac{U}{2}\sum\limits_{i}n_i(n_i-1)-\mu\sum\limits_in_i~~~~~~~~~(1)$$ For this we plot phase ...
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45 views

Excitations of a quantum spin-liquid

I have a simple (yet perhaps profound ?) question related to the physics of quantum spin liquids (QSL). These states of matter are "defined" as state that do not break any symmetry of the Hamiltonian ...
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2answers
347 views

Mean field theory = large-N approximation?

Wikipedia entry of 1/N expansion (or 't Hooft large-N expansion) mentions that It (large-N) is also extensively used in condensed matter physics where it can be used to provide a rigorous basis ...
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54 views

Is that possible to derive Landau-Fermi liquid theory from microscopic equation?

The question arised from reading Wen's book "Quantum Field Theory of Many-body Systems (Oxford 2004)" p204 To appreciate the brilliance of Landau-Fermi liquid theory, let us look at the ...
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955 views

How does the speed of electrons change around a circuit?

I have been thinking about ways of teaching electronics and I'm wondering if the following is true... For starters, when we talk about voltage as energy per unit charge, is this energy manifest ...
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47 views

Ising formulation of the graph isomorphism problem

Context I have been going through Ising formulations of many NP problems by Andrew Lucas. In section 9 on page 22, the author gave an Ising formulation of the graph isomorphism problem. Here I ...
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1answer
58 views

How does band gap vary with the cell volume?

How does band gap vary with the cell volume? is there a relation? If the volume is compressed, the interaction between atoms would be more, therefore the perturbation is higher hence the splitting ...
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80 views

Question about bosonization method

I have a question about bosonization method used in 1D system. Generally the bosonized field is assume to the following form \begin{equation} \psi = e^{i\phi}, \quad \phi = \phi^\dagger ...
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130 views

A formula in Sung-Sik Lee's paper

I want to ask if anyone has gone through the derivation of the second equality in the following formula which comes from http://journals.aps.org/prb/abstract/10.1103/PhysRevB.80.165102.
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1answer
35 views

Neutron Diffraction Device

I was just wondering if we have a beam of neutrons, lets say of various energies meeting or colliding with a powder of monoatomic crystallites, what would happen to the neutron beam and the energies ...
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458 views

Does electron-electron scattering contribute to resistivity?

Electron-phonon and electron-defect scattering clearly contributes to resistance, but pure electron-electron scattering conserves the total momentum (and energy) of all the electrons. Then, how is it ...
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167 views

List of experimental band gap

does anyone know where one can find a list, database, book of experimental bandgap values of semiconductors? Is there such a collection? Or do I need to scour papers one at a time to get the values?
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262 views

Degenerate perturbation theory applied to topological degeneracy?

Consider a quantum system described by a gapped Hamiltonian $H_0$ with degenerate ground states (GS), adding a perturbation term $V$ to $H_0$, then the low-energy physics can be described by an ...
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44 views

How to distinguish Bose glass and superfluid phases in a harmonic trap?

In mean-field study of Bose-Hubbard model in an optical lattice, what parameter can be calculated to distinguish Bose glass and superfluid in a harmonic trap?
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2answers
95 views

Is an object's color/shine/texture dependent on its electrons only? If electrons are same then why are there so many different colors?

when we look at an object be it a metal or a non-metal are we looking at its electrons only, so then if all electrons are same then why do different chemicals or elements or objects have different ...
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487 views

When can we take the Brillouin zone to be a sphere?

When reading some literatures on topological insulators, I've seen authors taking Brillouin zone(BZ) to be a sphere sometimes, especially when it comes to strong topological insulators. Also I've seen ...
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1answer
1k views

s-wave, p-wave or d-wave collisions in scattering theory

In scattering theory, what is a good intuitive picture to think of s-wave, p-wave or d-wave collisions ? What is their importance and what are the examples where a particular one is assumed to be the ...
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110 views

Why is an optical magnon with k=0 not an eigenenergy state?

I found in a paper the following explanation. Unfortunately, I can't understand it. Can anyone help me on this? In the limit of equal spins an optical magnon with k=0 gets an acoustical one at the ...
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2answers
116 views

Is the magnetization direction of a permanent magnet fixed?

I am now playing with a permanent magnet made of Neodymium. It is impressively strong. A question is, is the magnetization direction of the magnet fixed relative to its crystal structure?
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47 views

Physical, intuitive reason for divergence of dielectric constant at electronic percolation transition?

Several papers such as this (warning, PDF) and this (PDF again) talk about how, near the electronic percolation transition for a metallic 2D film, the real part of the dielectric constant diverges ...
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261 views

How to understand topological order at finite temperature?

I have heard that in 2+1D, there are no topological order in finite temperature. Topological entanglement entropy $\gamma$ is zero except in zero temperature. However, we still observe some features ...
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1answer
250 views

Definition for Chiral Spin Liquid

What is the definition of chiral spin liquid? Especially what does chiral mean here? I encounter a lot of terminologies with chiral. It seems they mean differently in different contexts. If you could ...
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780 views

Topological insulators: why K-theory classification rather than homotopy classification?

I am reading a 2009 paper by Kitaev on K-theory classification of topological insulators. In the 4th page, 1st paragraph in the section "Classification principles", he says, Continuous ...
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125 views

An approachable example of a field with a “mass gap”

Preamble: I have come to believe that alot of difficulties in explaining physics to people of all levels comes from the relatively mundane idea of a wave equation with a mass gap $$\left(-\partial^2_t ...
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1answer
101 views

Does the Night Mode of the screen display (LCD) save more energy?

In some cases, we can enable the Night Mode (reversing the bright and dark color of the display; such as White Text, Black Background) for the screen display. LCD(Liquid-crystal display) seems to be ...
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180 views

Difference between the 'phonons and virtual photons'

I understand what are virtual photons and the difference between the real and virtual photons. However, I am not able to clearly distinguish the difference between the 'phonons and virtual photons'. ...
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1answer
276 views

Energy band diagram for solid state: what is the meaning of $k$?

I am having trouble with the meaning of the $k$ vectors in an energy diagram. If I want to populate some band, let say using a laser, what will be the significance of $k$? Does it correspond to the ...
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2answers
109 views

Ashcroft Mermin Eq. 17.47ff

In "Solid State Physics" by said authors, Eq. 17.46 is $$ \rho^{ind}(\textbf{r}) = - e[n_0(\mu + e\phi(\textbf{r})) - n_0(\mu)]$$ and then the authors write In the present case we assume that ...
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1answer
450 views

What is tricritical point?

Critical point is the transition temperature of a second order phase transition. But what does tricritical point mean? WIki says that a tricritical point is a point in the phase diagram of a system at ...
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144 views

Material implementations of the holographic principle

I'm afraid this question is a little too open-ended, but bear with me while I find a better formulation. carbon allotropes (like fullerenes and graphene) are regular patterned. Conduction bands of ...
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15 views

Why do ferromagnetic thermocouples change their behaviour near the Curie point?

Having read Wikipedia's current explanation that certain types of thermocouple (specifically Type K thermocouples) experience a step-change in their potential when they reach the Curie point of the ...
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1answer
61 views

Locality in Condensed Matter Lattice Model

What is a proper definition of locality in condensed matter lattice model? I emphasize "condensed matter" because there is no Lorentz symmetry or "speed of light". I think it is quite important ...
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258 views

Are there new states of matter at ultrahigh temperatures and densities?

Under extreme energetic conditions, matter undergoes a series of transitions, and atoms break down into their smallest constituent parts. Those parts are elementary particles called quarks and ...
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124 views

What are the current contenders/most promising approaches to High Tc Superconductivity?

I want to know what kinds of things theorists are currently looking at. Specifically, I want to know more about the promise that field-theoretic methods are showing. I am studying superconductivity at ...
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4answers
3k views

Why is the canonical ($NVT$) ensemble often used for (classical) molecular dynamics (MD) simulations?

Molecular dynamics (MD) simulation is a common approach to the (classical) many-body problem. It relies on integration of Newton's equations of motion to simulate the trajectories of many (e.g., ...
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82 views

Graphene Chern number for Dirac nodes

Why do we add winding number at two Dirac nodes to determine topological phase?
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2answers
558 views

Subtleties in the exact solution to the 1D quantum XY model, in particular the Bogoliubov transformation

I am writing programs to construct the spectra of models with known exact solutions, and soon noticed some subtleties that are not often mentioned in most references. These subtleties are not ...
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1answer
69 views

Interchaging boson and fermion on an infinite 1 dimensional line

In 1+1 dim bosonization, one introduce the Klein factors, which are Hermitian and satisfies Clifford algebra. (1) In the case of 1 dim space is a 1D ring ($S^1$ circle), then one have left-right ...
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107 views

Mean-field theory : variational approach versus self-consistency

I have a general question concerning mean-field approaches for condensed matter classical of quantum statistical mechanic systems. Does determining the mean-field by a variational approach always ...
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28 views

How to isolate and verify the source of optical absorption/effects?

This is similar to my other question, but not the same -- that one was about the energy ranges of various absorption mechanisms, and this one is more about experimental techniques to find them. Let's ...
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37 views

Condensate fraction and momentum distribution in cold atom experiments

In ultrcold atom experiments, people usually use time-of-flight technique to measure the momentum distribution, then get the condensate fraction information from the interference pattern. What is the ...
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1answer
59 views

What does it mean for electrons to be “diffusive”?

I'm reading this paper and it has the line (end of 3rd paragraph, page 2): It turns out that the simple fact that electrons are diffusive instead of freely propagating leads to a profound ...
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3answers
1k views

What does an electron's wavevector mean inside of a crystal?

With a plane wave, I always took the direction of the wavevector, $k$, as the direction of propogation (magnitude proportional to the inverse wavelength). Alternatively, it could represent the ...
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63 views

Proximity effect and integrating out the quasiparticle degrees of freedom

I am reading at the moment the paper http://arxiv.org/abs/1401.5203 and try to reproduce the results. One result is the proximity correction $S_{\Sigma}$ to the system $$ e^{-S_{\Sigma}} ...
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127 views

Should a polyatomic crystal behave similarly to the bulk of each/either of its constituent elements?

Generally, metals are usually fairly conductive, but their oxides aren't. I know conductivity is just one attribute, but in general, should you expect a, say, diatomic bulk crystal's properties to be ...
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68 views

Neel order and O(3) model

The coarse grained fluctuations of the Neel order parameter in the half integer spin anti-ferromagnetic Heisenberg model is described by the O(3) non-linear sigma model with a strange berry phase ...
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94 views

Topolgical insulators order parameter

For topological insulators Is there any way to define order parameter for topological phase transitions?
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35 views

Chern bands and HEP Lattice Fermions: the emergence and the exact map

Chern bands or Chern insulators in 2 spatial dimensional(2D) are a way to construct the bulk insulating gap, but with edge or surfaces with gapless fermions. Such gapless fermions are emergent, and ...
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196 views

No Lagrangian description v.s. No quasi-particle description

This post is aimed to stimulate some discussions. We are familiar with many physical descriptions and theories of the (many-body quantum) system, with both quasi-particle description and Lagrangian ...