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

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Fermion 1D Hubbard Model ground state in the U = 0 limit

I am trying to determine the ground state of the 1D fermionic Hubbard model at half-filling of $2L$ sites with $L$ electrons with spin-$\uparrow$ and $L$ electrons with spin-$\downarrow$ in the $U=0$ ...
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37 views

electron-electron interactions in 1-D electron gas

The electron-electron interaction contribution to the hamiltonian in $k$-space representation is given by ...
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1answer
114 views

Flakes of Ice in Frozen Milk and Orange Juice

When I was a kid, my family used to put our gallon jugs of milk and orange juice into the freezer when we'd go away on vacation so that they would keep longer. As I remember it, if we were gone for a ...
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50 views

Weaker Formulations of Bulk-boundary Correspondence for Interacting Systems

From this post, it seems that bulk-boundary correspondence does not hold in general for interacting systems. What is meant by bulk-boundary correspondence there appears to be the existence of robust ...
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1answer
41 views

Metallic to insulating spin density wave transition in Hubbard model

For a half-filled Hubbard model with weak on-site Coulomb interaction ($U/t<<1$), it's quite intuitive that very likely the system will be in metallic phase. However, there is also such a ...
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1answer
85 views

What has been observed as the “Hawking radiation” emitted by a black hole analog?

I've noticed this paper which explains that they have observed the "Hawking radiation" emitted by a black hole analog. In which sense the Bose-Einstein condensate described by the paper can be ...
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61 views

What is meant by time reversal invariant momenta (TRIM) point of the Brillouin zone (BZ) and how to determine them in 3D reciprocal space?

In literatures related to topological insulators, I have frequently encountered the term "time reversal invariant momenta" or TRIM points in the Brillouin zone of the crystal. In many literatures, I ...
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2answers
538 views

What does Fermi level in the band gap mean?

What does it mean that the Fermi level for some semiconductors lie in the band gap? Is Fermi level definition different from what is know as usual? We define the Fermi level as the highest level of ...
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26 views

Charge density waves: site-centering v.s. bond-centering

Question about charge density wave (CDW): From this Ref. page 13, why bond-centering charge density wave is naturally compatible with the observed coexistence of charge ordering and ...
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25 views

Spontaneous breaking order and the Peierls order

From this this Ref, several types of orderings are considered. Question: What are the Hamiltonians which support the Peierls order? Do they necessarily break translational symmetry or break the ...
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23 views

Nuclear spin relaxation, quasi-particle energy and spin spectral density

Below is a measurement of the longitudinal nuclear spin relaxation ($1/T_1$). Ref: Fig 4 of page 24 Competing ground states in low dimensions. My question concerns the statement in this Ref that: ...
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30 views

flux quantization in super conductor

Magnetic force is a centripetal force. By that, we can make $$B = (h / \lambda r q) \, .$$ Is this flux quantization in superconductor ($h/q$)? If it is, is the wavelength of an electron the ...
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65 views

What theory describes high temperature superconductivity more sucessfully?

We know that there are so many theories on the high temperature superconductivity in cuprate. E.g. the U(1)/SU(2) gauge theory description of doped Mott insulator[Lee, Nagaosa, Wen], the phase-string ...
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2answers
59 views

What can we learn from a band structure diagram?

Other than the band gap and its magnitude, what are the things that we can immediately learn about the properties of the material just by glancing at its band structure? Can we say something about ...
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10 views

Which program to use to draw condensed matter phase diagrams? [duplicate]

I have data in three columns: first two columns give us the (x,y) coordinate of different points and third column gives us the phase of the physical state represented by that specific point. Now, ...
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1answer
110 views

Can interacting Hamiltonians always be written in second quantized form?

Is it always possible to write interacting Hamiltonian in a second quantized matrix form like we do it for non-interacting form $$H=\sum _{\alpha\beta}C_\alpha^\dagger h_{\alpha\beta} C_\beta$$ ...
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38 views

Topological S-matrix as an operator in the graphical calculus

My question comes from the following classic paper by Kitaev: Anyons in an exactly solved model and beyond (arXiv link) In Appendix E (pg 86), Kitaev introduces a diagram operator $S_z$ which acts ...
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28 views

Effective mass of almost ideal fermi gas

I am trying to reproduce this famous result of effective mass of almost ideal fermi gas(Galitskii 1958 The energy spectrum of a non-ideal Fermi gas). There are two kinds of ways to find effective ...
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34 views

Connected diagrams in Sine-Gordon action

I consider a bosonic action of the type $$\int dx d\tau\left( \underbrace{a(\nabla\theta)^2+b(\nabla\phi)^2}_{free} +\underbrace{c\cos{4\phi}}_{interaction}\right),$$ and want to treat the cosine term ...
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1answer
36 views

About characteristic lengths

I am reading about mesoscopic characteristic lengths.But I am not able to distinguish between phase coherence length $L_{phi}$ and inelastic length $L_{in}$. please tell me the difference and ...
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1answer
71 views

What are “two-centre integrals”?

Reading through some condensed matter physics literature I came across the term "two-centre integrals". Could someone explain what is meant by this in general? CONTEXT: "the overlap matrix and the ...
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3answers
171 views

Spectral properties in Solid state physics

So assume we have a periodic 1d Schrödinger operator $$- f'' + V(x) f(x)= \lambda f(x)$$ and we want $V$ to be periodic. Now if we assume that we are on a finite interval and that we have periodic ...
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47 views

Berry curvature and linear response functions

Let $\hat{A}^i (i = 1, . . . , n)$ be a set of hermitian observables and $F_i$ a corresponding set of external fields that are linearly coupled to $\hat{A}^i$. Starting from the ground-state at $F_i = ...
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21 views

Conductivity from Boltzmznn Equation for a metal in electric field

I'm trying to show that conductivity of a metal in uniform Electric field is: $$ \sigma=\int \frac{d\textbf{k}}{4\pi^3}\left (- \frac{\partial f}{\partial \epsilon} \right )\textbf{v(k)u(k)} $$ where ...
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78 views

Green function for single impurity

I am working on the first problem on self-consistent T-matrix approximation in Chapter 5 of Condensed Matter Field Theory by Altland and Simons. This is on page 234 of the textbook. I have some ...
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1answer
67 views

Negative Capacitance in Ferroelectrics

From the Devonshire theory of ferroelectrics we can obtain Polarization vs. Electric Field curve at a given temperature. From the graph it can be seen that a portion of the curve has negative slope ...
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1answer
1k views

What is the Difference Between a Type-1 and a Type-2 Superconductor?

As the title says, I was wondering what the difference was between a Type-1 and a Type-2 Superconductor. Especially in terms of the Coherent Length and Penetration Depth of a Magnetic Field - and how ...
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54 views

Why do we use the massive dirac fermion model for MoS2?

I can derive the massive Dirac fermion Hamiltonian using a tight binding model of graphene with a staggered sublattice potential, but many (including Xiao et al, PRL 2012) use this model for MoS2 as ...
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1answer
57 views

Green function for interacting system

If we can diagonalize our interacting Hamiltonian then can we write a Green's function like we do for a non-interacting system? Green's function here means Matsubara in frequency-momentum space, ...
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1answer
73 views

Proof of Kohn's theorem

In 1961 W. Kohn's paper ( Phys. Rev. 123, 1242 (1961) ) first stated that the electron-electron interaction does not change the cyclotron resonance frequency in a bulk three dimensional gas. I can ...
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29 views

About gauge in QHE

I have a 2D geometry with 4 leads in a square lattice structure. Please tell me how should I apply gauge to such a system that hopping term is translational invariant in the laeds in both directions ...
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1answer
154 views

What is the Difference Between BCS Theory and Ginzburg-Landau Theory?

What is the Difference Between BCS Theory and Ginzburg-Landau Theory? I have been studying Superconductivity and I know that Both of the theories (BCS Theory and Ginzburg-Landau Theory) can be used ...
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1answer
24 views

Exciton in semi-conductor

I don't understand why an exciton describes only the interaction between an electron hole and an electron in the conduction band? How is this interaction different from the interaction between an ...
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1answer
136 views

Hartree-Fock correction to $e$-$e$ interaction

The corrections to the energy per electron in a jellium model (uniform distribution of positive ion charge approximation to the regulated long range order ionic array) is given by (in units of Ry) ...
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33 views

Validity of the static limit of a dielectric function

In general, the dielectric function $\epsilon(q,\omega)$ reflects the spatial and temporal response of a condensed matter system to an applied potential. If we put an electron into an electron sea, ...
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1answer
176 views

Why is the ground state energy of the Heisenberg XXZ Model unbounded for some values of $J$?

At the moment, I'm looking at numerically studying the Heisenberg XXZ model. The Hamiltonian is given below: $$ H=\sum_{j=1}^{N-1}\left(J S_j^z ...
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1answer
53 views

Fast and slow modes, and the vanishing of certain diagrams during re-normalization

In the middle of pg. 452 of Atland and Simonss Condensed Matter Field Theory, they state the following: Terms of $\mathcal{O}(\phi _{\text{s}}^3\phi _{\text{f}})$ do not arise because the addition ...
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2answers
53 views

Why is 2-Dimensional Electron Gas (2DEG) materials interesting?

I noticed lots of research publications about 2DEG materials, e.g. AlGaN/GaN hetero-structures. I want to know what is interesting here. Is there a fundamental physics or technological potential?
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1answer
127 views

Origin of High-temperature Superconductivity

What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 25 kelvin?
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188 views

Speed of electrons in resistors

What affects the speed of electrons in a resistor? If two resistors are connected in series, they both have the same current; same number of electrons passing at a point per second. Suppose one ...
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39 views

Is there an analytical expression for the conductivity of the surface of topological insulators?

I have a question about the conductivity on the surface of Topological Insulators (TI): Is it accurate to model the conductivity by the Drude model (I read a paper that modeled the conductivity with ...
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46 views

Can we use combined symmetry to simplify the calculation of algebraic PSGs?

In classifying mean-field spin liquids under projective construction, the algebraic projective symmetry group (PSG) approach focus on the mathematical construction of the possible extensions of the ...
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1answer
149 views

Why does graphene exist?

I started to read some articles on graphene and almost all say that graphene was discovered late because physicists thought it would be unstable. Despite this, I didn't found a clear explanation of ...
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54 views

Reason of band inversion in graphene in kane and mele model

I am trying to simulate the graphene zigzag nano ribbon model of kane and mele. But I dont know how the inversion of band is taking place in it.Can anyone please suggest me about this .What makes this ...
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43 views

Why does Pressure Increase the Tc (Critical Temperature) of a Superconductor?

Just a heads up - please make this answer understandable to around 1st year degree level physics - not PhD research. So I can understand it - thanks. I was wondering why they Critical Temperature ...
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1answer
71 views

Why isn't doped silicon a strongly correlated electron system?

Most books on strongly correlated electrons claim that when the number of itinerant electrons is small and the screening length is large, that the system becomes "strongly correlated", (i.e. the ...
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1answer
96 views

Does the surface topological order on the boundary of 3D topological insulator also have topological ground state degeneracy?

The boundary of a 3D topological insulator can be fully gapped (under strong interaction) by the surface topological order without breaking the symmetry (see Fidkowski-Chen-Vishwanath, ...
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2answers
98 views

Why does the “counting rule” of band theory fail to predict the conduction properties of some materials?

I'm a little confused by the description I commonly hear about the electron counting rule in band theory. The general statement I find is that a "solid with an odd number of electrons per unit cell ...
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1answer
118 views

How legit is this paper claiming to have observed Hawking radiation? [closed]

I recently stumbled upon this paper. In it, the author claims to have found a signature for Hawking radiation in a condensed matter system. I know that experimentalists have been trying to find ...
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4answers
217 views

How does current flow in superconductors if Cooper pairs have zero momentum?

I've been reading a lot of condensed matter textbooks, which state both that the net momentum of a Cooper pair in a superconductor is zero, and that Cooper pairs have momentum when they carry current. ...