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

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Is it possible to determine the universality class of phase transitions by just analysing symmetry?

Since phase transition is closely connected with symmetry, I am wondering whether it is possible to determine the universality class of phase transitions just by symmetry? Actually, I found it is ...
0
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2answers
571 views

Quantum Quench Problem

I read about the quantum quench problem in condensed matter physics. But what does really mean? Has anybody a good explanation about the origin of quantum quench problem?
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1answer
227 views

What happens when molten metal cools within a strong magnetic field?

What happens when molten metal cools within a strong magnetic field? I don't know what more say, don't remember anything relevant to this in my uni text books.
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1answer
101 views

Can excitons be understood in terms of classical quantum physics?

From what I understand, an exciton is an electron-hole pair in a semiconductor that exists in a bound state (through the electrostatic potential). I have seen it stated that this pair behaves in a way ...
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2answers
313 views

Derivation of Lagrangian density for an infinite classical dielectric in interaction with the EM field

I am tasked with reading and reproducing all the steps in J.J. Hopfield's 1958 paper "Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals". Embarrassingly I am stuck ...
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2answers
475 views

Understanding Wikipedia's “Semiconductor Band Structure” diagram where the bandgap appears to increase with increasing density of states

I'm having a bit of trouble understanding the semiconductor band gap diagram on Wikipedia: (from Band gap article). Why is the size of the band gap increasing with the Density of States (DOS) in the ...
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1answer
81 views

Frequency dependence of permittivity — why not monotonic?

I naively thought that most materials were transparent to radiation of frequencies above their plasma frequency, and opaque to radiation below it. The most intuitive (and analyzed lightly in ...
4
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1answer
143 views

Topological Insulators: is HgX a special case?

I got confused by reading this article: Francois Virot, Roland Hayn, Manuel Richter, and Jeroen van den Brink. “Engineering topological surface-states: HgS, HgSe and HgTe.” arXiv preprint ...
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1answer
218 views

Why does total spin conservation law forbid the spin wave gap in Heisenberg magnets?

What is the explanation for total spin conservation forbidding the spin wave gap in Heisenberg magnets?
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450 views

When is quasiparticle same as elementary excitation, and when is not?

Can anyone shed light on the comparison between these two concepts?
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1answer
92 views

Two particles state of a 1D massive scalar field

Perfectly localized states are not normalized so do not belong to the Fock space (they belong to the rigged version). Suppose we approximate localized states with gaussians, what is the mathematical ...
6
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1answer
266 views

Why Landau Level quantization is observed only in low temperature and strong magnetic field in real experiment?

I know that Quantum Hall Effect and Fractional Quantum Hall Effect origin from Landau Level quantization. In magnetic field, the energy of in-plane(plane perpendicular to magnetic field) degree of ...
4
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1answer
1k views

Why there is a flat band for Kagome lattice?

For the nearest neighbor hopping model on the Kagome lattice, there is a flat band among the three energy bands. Is there some reason, such as symmetry or the special structure of the model, to ...
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1answer
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 ...
5
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1answer
299 views

Topology and Majorana bound states

I'm working at the moment on Majorana Bound states and their topological properties. Now I have a question about it. The Altland-Zirnbauer symmetry classes says us how many topological different ...
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0answers
60 views

What is the definition of a charge-neutral operator?

What is the definition of a charge-neutral operator? I guess it means something like: it is invariant under charge conjugation. It that correct?
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1answer
95 views

What is the universality class of transition in one-dimensional XXZ Heisenberg at $\Delta$ = -1?

In the one-dimensional spin-$\frac12$ XXZ Heisenberg model, $$H=J\sum_i{S_i^x S_{i+1}^x + S_i^y S_{i+1}^y+\Delta S_i^z S_{i+1}^z},$$ with $J>0$. There are two transition points: $\Delta=1$ ...
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2answers
248 views

Is the spin-singlet state also a Resonating-Valence-Bond(RVB) state?

The spin-singlet state of a lattice spin-1/2 system is defined as $S_x\Psi=S_y\Psi=S_z\Psi=0$, where $S_\alpha=\sum S_i^\alpha(\alpha=x,y,z)$ are the total spin operators, in other words, a ...
2
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0answers
130 views

Questions on the elementary excitations in the resonating-valence-bond(RVB) states?

It is known that the RVB states can support spin-charge separations and its elementary excitations are spinons and holons. But it seems that there are some different possibilities for the nature of ...
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0answers
32 views

Is there a difference in binding energy between a regular material and a doped one?

Say Silicon and boron doped silicon. Would the doping affect the binding energy? Could I see this in an XPS spectra?
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0answers
434 views

Exact diagonalization by Bogoliubov transformation

I am developing a model of multiple gaps in a square lattice. I simplified the associated Hamiltonian to make it quadratic. In this approximation it is given by, $$ H = \begin{pmatrix} \xi_\mathbf{k} ...
0
votes
1answer
273 views

Semiconductors: why the mass action law is not valid for very low temperatures?

I thought that it was valid for very low temperatures since for it to be valid I think that it is necessary to be in the non-degeneracy condition, which requires $E_G >> k_B T$ (with $E_G$ being ...
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1answer
679 views

low frequency permittivity of metals from Drude's model

In reference to the Optical constants of noble metals: the Drude model for microwave modelling regarding Drude's model these parameters were listed $$ \omega_P=1.36\times 10^{16} \text{ rad/s} $$ $$ ...
4
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0answers
118 views

Third-order topological quantum phase transition in p+ip superfluid

A two-dimensional spinless non-relativistic p+ip superfluid undergoes a quantum phase transition between the BCS (weakly-coupled) and BEC (strongly-coupled) regimes. This transition is driven by ...
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2answers
89 views

Bulk modulus of Liquid helium and first sound

Does anyone know where to find the bulk modulus of liquid helium ? I've been looking all over the internet but everywhere I get N/A. Any tips ? I'd need it to estimate the speed of first sound in ...
2
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0answers
140 views

Typical time scales for spin dynamics and lattice vibrations in magnetic solids

In a paper from the 1990s ([1]) on magnetovolume effects in ferromagnets, it is written that in most real situations, the moment (or spin) autocorrelation time is much larger than the period for ...
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2answers
130 views

Approaches to Fault tolerant quantum computation

What are the various approaches to fault tolerant quantum computation ? Two examples are 1. topological quantum computation which uses topological phases in quantum states (2-Dimensional for ...
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0answers
199 views

More physical explanation of impurity energy levels in a doped semiconductor?

I'm reading about doped semiconductors in Ashcroft and Mermin. They tell you that when donor impurities are added to a semiconductor, their energy level $E_d$ is just slightly below the conduction ...
3
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1answer
179 views

Why does water ($\mathrm{H_2O}$) only have two distinct fluid phases?

Water (and other substances) can exist in many distinct solid phases (with different crystallic micro-structure), but only in two fluid phases - liquid and gaseous, in which the molecules are oriented ...
3
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2answers
321 views

Why does the $\pi$-flux state have time-reversal symmetry?

It's known that the $\pi$-flux state of the antiferromagnetic Heisenberg model on the square lattice is an important concept. The $\pi$-flux state is described by the (simplified) mean-field ...
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1answer
386 views

Determining spectra of edge states numerically

Normally we write a Bloch Hamiltonian $H(\mathbf{k})$ for the bulk and determine the spectrum which gives us various bands i.e we basically obtain $E=E(\mathbf{k})$ for the bulk only. Also in the ...
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1answer
358 views

Difference between primitive unit cell and the associated basis?

As I understand it, the basis is the group of atoms whilst the primitive unit cell is the unit space that fits the total space without any gaps, and only containing one lattice point? How do the two ...
2
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1answer
178 views

Derive non-linear $\sigma$ model from a theory of SU(2) matirx

It's said in Chapter VI.4 of A. Zee's book Quantum Field Theory in a Nutshell, a theory defined as $L(U(x))=\frac{f^2}{4}Tr(\partial_{\mu}U^{\dagger}\cdot\partial^{\mu}U)$, can be write in the form of ...
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0answers
43 views

What's the necessary and sufficient condition for gauge equivalence in the projective construction?

The definition of gauge equivalence and notations used here is the same as those in my previous question. As we know, the condition $\chi_{ij}'=G_i\chi_{ij}G_j^\dagger$(where $G_i\in SU(2)$) is a ...
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0answers
44 views

Some question on the definition of flux in the projective construction?

Here I have some confusing points about the definition of flux in the projective construction. For example, consider the same mean-field Hamiltonian in my previous question, and assume the $2\times 2$ ...
2
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1answer
162 views

How many kinds of topological degeneracy are there?

Here I want to summarize the various kinds of topological ground-state degeneracy in condensed matter physics and want to know whether there exists any other kind of topological degeneracy. For ...
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1answer
226 views

A commutation problem in Hubbard model

Does the Hubbard Hamiltonian $$H=-t\sum_{\langle ij\rangle \sigma}c_{i\sigma}^{\dagger}c_{j\sigma}+h.c.+U\sum_{i}n_{i\uparrow}n_{i\downarrow}$$ commute with $\sum_{i}\mathbf{S}_i^2$? where ...
2
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0answers
89 views

Some questions on the Wilson loop in the projective construction?

Based on the previous question and the comment in it, imagine two different mean-field Hamiltonians $H=\sum(\psi_i^\dagger\chi_{ij}\psi_j+H.c.)$ and $H'=\sum(\psi_i^\dagger\chi_{ij}'\psi_j+H.c.)$, we ...
2
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1answer
188 views

What happens to chiral Majorana edge fermions near quantum phase transition in p+ip superconductors?

In the weakly-coupled BCS regime two-dimensional chiral (p+ip) spinless superconductors and superfluids posses a chiral gapless fermionic Majorana state localized near the boundary. This gapless edge ...
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1answer
158 views

Wave vector $\vec{k}$ vs position vector $\vec{x}$

My question is about the $k$-vectors in first Brillouin zone. If I am not misunderstood, the relation k = 2π/(Na) tells that when k goes to zero, we are very very far away from the reference atom and ...
6
votes
1answer
265 views

Goldstone mode in O(N) (non-linear $\sigma$ model)

The question is does the Non-linear $\sigma$ model have a Goldstone mode? Consider a $O(N)$ mode for which the Hamiltonian is $H=J\sum_{i,j}\vec{n}_i \cdot \vec{n}_j$, where ...
3
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0answers
142 views

Zumino's consistent and covariant anomalies - applied to quantum hall?

What is the `physical' meaning of consistent anomalies and covariant anomalies? Perhaps a good Reference is: Consistent and covariant anomalies in gauge and gravitational theories - William A. ...
2
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1answer
2k views

Valence band and conduction band, trying to get a clear picture!

I am trying to get a clear picture of the valence band, conduction band, and the band gap. Now I've been researching it for a little while now and understand most of what's going on. I'm still a ...
5
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1answer
498 views

Physical Interpretation of Relationship Between Hall Conductivity and Berry Curvature?

Why is the Hall conductivity in a 2D material $$\tag{1} \sigma_{xy}=\frac{e^2}{2\pi h} \int dk_x dk_y F_{xy}(k)$$ where the integral is taken over the Brillouin Zone and $F_{xy}(k)$ is the Berry ...
2
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0answers
666 views

What is the definition of particle-hole symmetry in condensed matter physics?

People often talk about particle-hole symmetry in solid state physics. What are the exact definition and physics picture of particle-hole symmetry? How to define the density of particles and holes?
2
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2answers
541 views

Probability of Different States - Canonical Ensemble - Partition Function

Consider a canonical ensemble of $N$ ideal gas atoms, which could have spin up or spin down. Why is it that the probability of finding the particle in a spin up state generally only involves the ...
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1answer
240 views

Dopant Charge Transfer and Fermi Level shift

When a system has a dopant, how much does the Fermi level shift? For example, say a finite concentration of substitutional dopants replace some bulk atoms, and each has one extra electron. Ignore any ...
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0answers
173 views

Average number of spin up particles

In a paramagnetic system, where $N = N_\uparrow + N_\downarrow$ is fixed, how does one calculate the average number of spin-up particles $\langle N_\uparrow \rangle$? You can assume we have the ...
2
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1answer
964 views

Partition Function for Two Level System

I have a system with $N_s$ sites and $N$ particles, such that $N_s >> N >> 1$. If a site has no particle, then there is zero energy associated with that site. The $N$ particles occupy the ...
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1answer
178 views

Is water a gas at critical density, room temperature?

I am quoting Chaikin, Lubensky, Principles of Condensed Matter Physics, p. 4. Now suppose we have a closed container of water vapor at a density of 0.322 g/cc at room temperature. As the ...