The study of physical properties condensed phases of matter, including solids and liquids.
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2answers
138 views
Graphene +1 extra carbon bond
I'm not a physicist just a curious mind, so please go easy!
I was just watching a BBC Horizon Documentary that featured a piece on the recently discovered material Graphene. One of the facts ...
5
votes
1answer
153 views
A question on the doped Kitaev-Heisenberg model?
Recently, some groups have studied the effects of doping the Kitaev model on honeycomb lattice(e.g.,http://arxiv.org/abs/1109.6681 and http://arxiv.org/abs/1109.4155) and their calculations show the ...
1
vote
1answer
52 views
Conceptual questions about Fermi surface
So I am wondering what kind of two dimensional Fermi surface is called quasi one dimensional, what is its character? Also, when there are orbital hybridization taking place in lattice site, what are ...
5
votes
0answers
120 views
How does Haldane conjecture follow from the topological $\Theta$ term
The one dimensional SU(2) Heisenberg quantum spin chain is known to be described by the 1+1d O(3) nonlinear $\sigma$ model with a $\Theta$ term, following the action
...
2
votes
1answer
154 views
Specific heat capacity of a 2D free electron gas
I have got so far the 2D density of states as $g(\epsilon)=\frac{Am}{\pi\hbar^2}$ where $A$ is the area of the "square" and $m$ is the the electron mass. Then I have found an expression for the the ...
2
votes
1answer
78 views
What states are satisfying an entropic area law and why do they satisfy it? More specificly why do matrix product states satisfy it?
I am currently reading some papers concerning the question why the density matrix renormalization group (DMRG) method is working well for simulating one dimensional systems and bad for higher ...
2
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2answers
72 views
Are electronic wavefunctions in band gap insulators localized? is a single-particle picture sufficient in this case?
I am having trouble understanding the physics of band gap insulators.
Usually in undergrad solid state physics one looks at non-interacting electrons in a periodic potential, with no disorder.
Then, ...
3
votes
1answer
97 views
Fermi level with Landau levels
So my question is regarding where the Fermi energy is when you have 2D electron gas in an applied magnetic field. My book explains that, using the Landau gauge, you find that the 2D density of states ...
0
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1answer
102 views
Calculation of Number Density
Number density equation is given by
$ n= \dfrac{(N_A)\rho}{M} $
where
$ N_A =6.023\times10^{23} mol^{-1} $
$ \rho=8.02\ g/cm^3 $(at 1500 degree celsius.)
$M=63.546*1.6605\times10^{-24} g$
Whats ...
2
votes
1answer
37 views
Does an Ising lattice that returns to equilibrium create a current by induction?
Consider you have an Ising lattice with a dominant up component out of thermal equilibrium, that's your initial state. The down spins want to flip up and align with the ups, and they'll do so until a ...
6
votes
2answers
149 views
Why does the quantum Heisenberg model become the classical one when $S\to\infty$?
The Hamiltonian of the spin $S$ quantum Heisenberg model is
$$H = J\sum_{<i,j>}\mathbf{S}_{i}\cdot\mathbf{S}_{j}$$
I have read that when the spin quantum number $S\to\infty$, quantum fluctuation ...
5
votes
2answers
158 views
A question on the existence of Dirac points in graphene?
As we know, there are two distinct Dirac points for the free electrons in graphene. Which means that the energy spectrum of the 2$\times$2 Hermitian matrix $H(k_x,k_y)$ has two degenerate points $K$ ...
5
votes
1answer
215 views
A question about Haldane's conjecture
Haldane's conjecture states that the integer spin antiferromagnetic Heisenberg chains have a gap in the excitation spectrum. However, the dispersion relation of the antiferromagnetic spin wave is ...
7
votes
1answer
90 views
Phase transition water
The water-gas phase transition is said to be similar to the ferromagnetic-paramagnetic phase transition (same set of critical exponents = same universality class). In the former case the order ...
0
votes
1answer
70 views
Momentum change in colisions (Drude Model)
A particle suffers elastic colisions with scattering centers with a probability of colision per unit time $\lambda$. After a colision the particle is in a direction caracterized by a solid angle ...
0
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1answer
150 views
Photon Absorption and Emission: Conductors v. Semiconductors
I'm having a hard time understanding how photon absorption and emission in metals (conductors) compares to semiconductors. Obviously, in SCs, absorbed photons lead to electron-hole pairs and emitted ...
2
votes
0answers
60 views
Why do Fermi liquids have T^2 resistivity?
I have often read that metals that are Fermi liquids should have a resistivity that varies with temperature like $\rho(T) = \rho(0) + a T^2 $.
I guess the $T^2$ part is the resistance due to ...
2
votes
1answer
98 views
How to define the order parameter of the q-state Potts model?
The order parameter of Ising model can be defined as $m=\frac{N_1-N_2}{N}$, if $N$ is the total number of lattice points, $N_1$ and $N_2$ is the number of lattice points spin up and down respectively, ...
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0answers
91 views
In what direction does a frustrated magnetic moment get aligned?
Consider 3 layers of Ferromagnetic materials stacked on top of each other with appropriate spacer layers in between. Let the top and bottom layers be pinned to layers of Anti Ferromagnets adjacent to ...
5
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0answers
234 views
Do EM waves transmit spin polarization?
Suppose you have a normal dipole antennae (transmitter and receiver) . Spin polarized current (as opposed to normal current) is sent into the transmitter, it emits an EM wave and the Receiver receives ...
0
votes
0answers
96 views
Wave function ansatz for disclinated graphene with spin
I am currently investigating spin dynamics in disclinated graphene. More information about my approach can be found in my other post. I would like to know if my approach is somewhat correct to find ...
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0answers
71 views
How to charge a field?
In a previous post [ Noether theorem, gauge symmetry and conservation of charge ] we were discussing the different ways to demonstrate the current conservation: via the first Noether theorem applied ...
3
votes
1answer
65 views
Spin Liquid in a band insulator?
In the literature, spin liquids are only possible in Mott insulators, however, I'm not entirely sure why the nuclear spin can't create a spin liquid in a band insulator.
Is this possible? If so, is ...
3
votes
1answer
155 views
Why does a superconductor obey particle-hole symmetry?
We normally solve the Bogoliubov-de Gennes (BdG) equations in order to compute the energy spectrum of a superconductor. The Nambu spinor is a common object that is used in formulating these equations. ...
8
votes
1answer
185 views
What is the “BCS Cooper pair condensation” as a physical phenomenon in terms of experiments?
"Thought" experiments and "numerical" experiments are allowed.
This question is motivated by the question Has BCS Cooper pair condensate been observed in experiment? ,
and by our recent research on ...
9
votes
3answers
330 views
Introduction to Anderson localization [closed]
I find Anderson's original paper too terse. I am looking for something that introduces me gently to the subject so that I can understand Anderson's paper and other literature. What references are out ...
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1answer
88 views
Flow of supercurrent in a superconductor
I have two questions one practical and one theoretical. Even though I have a decent understanding of superconductivity both phenomenological as we theoretical (i.e. BCS) some things just slipped ...
3
votes
1answer
107 views
Graphene with a disclination and the spin-orbit coupling
I am trying to follow the methods used in this paper (http://arxiv.org/pdf/1208.3023.pdf) to construct the Hamiltonian of a graphene cone, but taking into account the spin-orbit coupling.
The paper ...
4
votes
1answer
202 views
Is edge state of topological insulator really robust？
I am a little confused! Some people are arguing that the gapless edge state of Topological insulator is robust as long as the time reversal symmetry is not broken，while other people say that it is ...
10
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2answers
482 views
Basic questions in Majorana fermions
Why any fermion can be written as a combination of two Majorana fermions? Is there any physical meaning in it? Why Majorana fermion can be used for topological quantum computation?
3
votes
2answers
186 views
What limits the maximum attainable Fermi Energy for a material experimentally?
Either through doping or gating. What are some good terms to search for if I'm looking for some experimentally obtained values for particular materials? I'm particularly interested in what the limit ...
0
votes
0answers
93 views
Ground and first excited state of non interacting spin system Hamiltonian
For a non interacting spin system containing two $\frac{1}{2}$ spin particles I am trying to determine its Hamiltonian. If the energy of a up spin is $+\mu {\bf B}$ and a down spin is $-\mu {\bf B}$, ...
0
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1answer
87 views
Ground states of the Hamiltonian of a two spin system
For the spin system shown in this graph (http://i.stack.imgur.com/3lg1R.png), the Hamiltonian is $$S^{(1)}_z\cdot S^{(1)}_z=\frac{1}{4}\begin{pmatrix}
1 & 0 &0 &0 \\
0&-1 &0 ...
3
votes
1answer
102 views
Why FQHE need a lower energy state?
There are a lot papers explaining why Laughlin's wavefunction are energetically favorable, but seldom explain why a lower energy state could explain the plateau at $\nu=1/3$. I met at several places ...
2
votes
0answers
64 views
Electron Relaxation/Polarization for and n-type Semiconductor
Please help me understand the following (general) statement, referring to electrons in a full valence band of an n-type semiconductor:
"An electron filling up the last empty state in the valence band ...
1
vote
2answers
99 views
Eigenfunctions in periodic potential
For Hamiltonian $\operatorname H$ and lattice translation operator $\operatorname T$, if
$$\operatorname H\psi=E\psi, \qquad \operatorname T\psi=e^{ik\cdot R}\psi,$$
and
$$\operatorname ...
1
vote
1answer
114 views
Hamiltonian of a simple graph
I have a spin system:
As shown in the picture, there are two spins S1 and S2, and a pair of interactions between them. One is a ferromagnetic interaction and the other is anti ferromagnetic ...
3
votes
2answers
148 views
Difference between Monte Carlo and Quantum Monte Carlo methods?
What are the differences between Classical Monte Carlo methods and Quantum Monte Carlo methods in condensed matter physics? If one want to study strongly correlated systems with Quantum Monte Carlo ...
0
votes
1answer
60 views
The orthogonalized plane waves
An orthogonal plane wave with wave number $k$ is written as
$$ OPW_k=e^{ ik\cdot r}-\sum_\alpha \psi_\alpha(r) \int \psi^*_\alpha (r'') e^{ik\cdot r''} d\tau'',$$
where index $\alpha$ and $k$ ...
1
vote
1answer
282 views
Why path integral approach may suffer from operator ordering problem?
In Assa Auerbach's book (Ref. 1), he gave an argument saying that in the normal process of path integral, we lose information about ordering of operators by ignoring the discontinuous path.
What did ...
5
votes
2answers
99 views
Toric Code and Random Bond Ising Model
It was established by Dennis, Kitaev et al. that the 2D Toric Code
can be mapped to a 2D Random Bond Ising Model. The original derivation
was given in the paper "Topological quantum memory" which ...
11
votes
1answer
433 views
Emergent symmetries
As we know, spontaneous symmetry breaking(SSB) is a very important concept in physics. Loosely speaking, zero temprature SSB says that the Hamiltonian of a quantum system has some symmetry, but the ...
5
votes
0answers
255 views
Is the “particle number” of “electrons” well defined in Wen's string-net theory of elementary particles?
According to professor Wen's string-net theory(Colloquium: Photons and electrons as emergent phenomena, Levin and Wen, Rev. Mod. Phys. 77, 871(2005), see e.g. http://arxiv.org/abs/cond-mat/0407140), ...
4
votes
2answers
115 views
Crystal Angular Momentum
In a crystal, we don't have full translational symmetry, but we still have discrete translations. This allows us to define "crystal momentum" that is conserved modulo a reciprocal lattice vector.
In ...
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vote
1answer
78 views
In the big crunch theory, when the big crunch singularity forms, can the resulting black hole decay through hawking's radiation?
I've been pondering about this and I couldn't really find the answer for this. The big crunch theory postulates that the universe will eventually stop expanding and reverse back in on its self into a ...
0
votes
0answers
20 views
What favors island growth of a sputtered material?
What would be the best choice of parameters in general if one would like to get pure island growth (i.e. Volmer-Weber growth) in a sputtering deposition process and what would be a good estimate of ...
2
votes
1answer
134 views
What happens to atoms inside the black hole?
Black holes have very high gravitational force intending to crush everything. So as we know atoms in a molecule have inter atomic spacing between then and further electron,s also revolve at a certain ...
6
votes
2answers
162 views
Is the speed of sound almost as high as the speed of light in neutron stars?
Have you ever wondered about the elastic properties of neutron stars?
Such stars, being immensely dense, in which neutrons are bound together by the strong nuclear force on top of the strong gravity ...
1
vote
1answer
69 views
Dynamic structure factor
Dynamic structure factor is the spatial and temporal Fourier transform of Van Hoves time dependent pair correlation function. It is written as
$$ S(k,\omega)= \frac{1}{2\pi}\int F(k,t)\exp(i\omega ...
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0answers
107 views
Phonon Momentum
I am reading Charles Kittel's solid state physics and wondering what's the mechanism that neutron waves and photons can interact with phonons and the process obey the generalized momentum-energy ...
