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

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136 views

Numerical analytic continuation for Green's function

Recently, I happened to hear about the possibility of doing analytic continuation numerically. That sounds attractive for the ubiquitous $\mathrm{i}\omega_n\rightarrow\omega+\mathrm{i}0^+$ procedure, ...
10
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1answer
541 views

Do we have magnetic monopole? Is charge, according to yesterdays paper on Nature by Ray-Roukokoski-Kandel-Möttönen-Hall (30.01.14) quantized?

As everybody here knows, Maxwell's equation would look more beautiful if a magnetig charge were present. Beyond the aesthetics question, if a single magnetic Dirac monopole would be found, the ...
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11 views

Effective medium approximation for calculating relative permeability of cylindrical shells of alternating magnetic/nonmagnetic layers

I'm trying to apply the effective medium approximation to my own research. http://ieeexplore.ieee.org/ielx5/20/30863/01430899.pdf?tp=&arnumber=1430899&isnumber=30863 As far as I understand, ...
2
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84 views

How to understand the equivalence between Andreev reflection and Cooper pair injection?

It is well know that Andreev reflection dominates the subgap transport at the normal metal-superconductor interface. An incident electron can be reflected as a hole in the Nambu space, which ...
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13 views

low magnetic permeability, low coercivity compounds?

First question: Does anyone know of any low permeability, low coercivity magnetic compounds? A physics professor told me that I should dig deep in the literature, but that I should look for materials ...
5
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250 views

Naive questions on Goldstone modes and a possible duality relation?

For example, let's consider a 1D spin-1/2 ferromagnetic (FM) Heisenberg chain $H=-J\sum_{i=1}^{N}\mathbf{S}_i\cdot\mathbf{S}_{i+1}$ with periodic boundary conditions. Now we want to study its low ...
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72 views

Why is the projective symmetry group (PSG) called projective?

As discussed by Prof.Wen in the context of the quantum orders of spin liquids, PSG is defined as all the transformations that leave the mean-field ansatz invariant, IGG is the so-called invariant ...
2
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1answer
104 views

Kane and Mele's argument on the existence of edge states in quantum spin Hall effect of graphene

Borrowing from Laughlin's argument on quantum Hall effect, Kane and Mele argued why there must be edge states in graphene with spin-orbit coupling in one paragraph, which is above the one with ...
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44 views

Any easily available ferromagentic material with $T_c$ in room temperature?

I want to experiment it in my house or office. I think it would be of great fun to see the transition in real life.
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8 views

Why are the bounds to the permittivity $\epsilon$ a circular arc in the complex plane?

I'm reading this paper which is essentially about connecting the complex permittivity $\epsilon$ with the microstructure of a thin film. They talk about how you can place limits on the possible values ...
2
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1answer
59 views

How does temperature affect an electrical current

Synopsis I have read an interesting article J. Halderman et al. "Lest we remember: cold boot attacks on encryption keys" in computer science regarding cold booting. The paper discusses how the use ...
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16 views

Generalization of a spin-glass order parameter definition

I am going through the paper, Solving the graph-isomorphism problem with a quantum annealer, by Hen et. al. Equation 4 on the second page gives the definition of the spin-glass order parameter as ...
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59 views

Difference of the O(N) Non-linear Sigma model and SO(N) Non-linearSigma model

The Hamiltonian \begin{equation} H=J\sum_{i,j}\vec{n}_i\cdot\vec{n}_j \end{equation} is invariant under a global rotation $\vec{n}_i\rightarrow R\vec{n}_i$, where $\vec{n}$ is a $N$ component rotor ...
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62 views

Two-Dimensional Tight-Binding Dispersion Relation

As in my last post, I am doing out a calculation in Giamarchi's Many-Body text: http://dpmc.unige.ch/gr_giamarchi/Solides/Files/many-body.pdf. This time, I am going through the derivation of the ...
2
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1answer
56 views

Hamiltonian for the Periodic Kitaev Model

The Hamiltonian for a system of spinless fermions on a 1D chain (with chemical potential $\mu=0$) is given by $$ H=-\sum_j\left( c^\dagger_{j+1} c_j+h.c.\right)+\Delta \sum_j \left( ...
2
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2answers
205 views

Difference between charge density wave and charge distribution

We can always see modulated charge density, the Friedel Oscillation, around an probe charge due to other electrons' response. Can this be called charge density wave (I believe not)? If not, what is ...
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22 views

For the gapped phase of Kitaev model, where does the first excited state reside in, the zero-flux sector or not?

As we know, there are both gapless and gapped ground states of the Kitaev model, and let's fix the couplings $J_x,J_y,J_z$ such that the model being in the gapped phase. My question is, does the first ...
3
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1answer
174 views

Naive questions on the ground states of Kitaev model

I got some naive questions on the ground states of honeycomb Kitaev model (with open boundary conditions): (1) Consider a simple case that $J_x=J_y=0$, then the model reduces to $$H=J_z\sum_{z\text{ ...
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42 views

What is the minimal symmetry required for a spin Hamiltonian to describe a spin-liquid ground state?

Let's restrict to the case of spin-1/2 system. As we know, a spin-liquid (SL) state is the ground state of a lattice spin Hamiltonian with no spontaneous broken symmetries (sometime it may ...
2
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1answer
83 views

Solving the BCS Hamiltonian via the Bogoliubov Transformation

I was doing a calculation in Giamarchi's Introduction to Many Body Physics, chapter 3, on BCS theory and second quantization, and ran into some confusion with the BCS Hamiltonian. The pdf is here for ...
5
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740 views

When is use of the 'effective mass' concept appropriate?

In textbooks the characteristic length scale of an exciton, or an electron bound to dopant atom, in silicon is calculated by analogy to the vacuum case. Bohr radius in vacuum: $$a_0 = \frac{4 \pi ...
7
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1answer
246 views

Can I integrate out the fermion field that is not gapped?

This piece of argument has been repeated again and again by experts, that is Since the fermions are gapped, then I can integrate it out. but I have no idea of what will happen if the fermions ...
2
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23 views

In k$\cdot$p theory, how does one calculate the bulk inversion asymmetry coefficients given in table 6.3 in Winkler?

In k$\cdot$p theory, how does one calculate the bulk inversion asymmetry coefficients given in table 6.3 in Winkler? Winkler's book on spin-orbit coupling effects is available free online. In ...
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20 views

Phase coherence and interference effects in Anderson localization

Anderson localization results from wave interference of the between multiple-scattering paths from random impurities, yielding wavefunctions with exponentially decaying tails and absence of diffusion. ...
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45 views

Higgs Boson and its relation to the vacuum

The Higgs boson and the electroweak theory used symmetry breaking from condensed matter physics as its inspiration. The BCS theory of superconductivity is one such condensed matter symmetry breaking ...
3
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1answer
142 views

Diagonalization of Hamiltonian

Typically, one way of understanding the physics of an interacting quantum system is by diagonalizing the Hamiltonian. In principle, can we always diagonalize a Hamiltonian, such that it is expressed ...
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57 views

Gutzwiller mean-field method in Bose Hubbard model

Gutzwiller mean-felid method is an efficient way to study Bose-Hubabrd model in optical lattice with a harmonic trap. Gutzwiller method assumes there is no spatial correlation within the trap, so ...
3
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1answer
144 views

Why are there gapless excitations in the anti-ferromagnetic Heisenberg model while the true ground state is a singlet?

The true ground state of the anti ferromagnetic quantum Heisenberg Model (nearest neighbor only)is known to be a singlet (I think this is Liebs theorem.) Since a singlet is invariant under rotations, ...
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1answer
66 views

Is linear momentum conserved in a system with open boundary conditions?

In a one dimensional lattice system with periodic boundary conditions, in which the last and the first site of the lattice are the same site, momentum is conserved modulo a vector of the reciprocal ...
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2answers
476 views

What is a Zero-Phonon Line (ZPL)?

I am trying to understand the electronic structure of the negatively charged NV centre in diamond, where there is a so-called Zero-Phonon Line (ZPL) in the spectrum. Can anybody explain what a ZPL is? ...
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22 views

Superfluid momentum distribution

Bosons confined in a optical lattice under some conditions can enter a superfluid phase. Momentum distribution can be measured using time of flight method. But why is there a sharp peak in momentum ...
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285 views

Understanding Elitzur's theorem from Polyakov's simple argument?

I was reading through the first chapter of Polyakov's book "Gauge-fields and Strings" and couldn't understand a hand-wavy argument he makes to explain why in systems with discrete gauge-symmetry only ...
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49 views

Question about Bose-Hubbard Model

By using an optical lattice, how can one change the interaction term $U$? And how is the superfluid phase achieved in the hard-core boson regime? Why are these phases identified as superfluid or mott ...
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25 views

Order of Monte Carlo integration and frequency summation

I am currently trying to calculate an integration formula of a linear response function by Monte Carlo method. It is a multiple integration over three 3D vectors, i.e., nine dimensions in all. And ...
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2answers
159 views

Why are Cooper pairs formed by electrons of opposite momentum and spin?

I understand that Cooper pair in low-temperature superconductivity are formed by electron-phonon interaction. Normally one then assumes that electrons of opposite momentum and spin are paired. This is ...
3
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1answer
183 views

Majorana wavefunction

I'm trying to compute the wavefunction for a Majorana state in an nanowire/superconductor hybrid system, like arXiv: Majorana Fermions and a Topological Phase Transition in ...
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48 views

Should Brillouin zone be a continuous object rather than a discrete one in the thermodynamic limit?

For example, just consider a 1D atom chain with $N$ sites and lattice constant $a=2\pi$, under periodic boundary conditions, the crystal momentum reads as $k=\frac{n}{N}\frac{2\pi}{a}=\frac{n}{N}$, ...
3
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1answer
216 views

Is the spin-rotation symmetry of Kitaev model $D_2$ or $Q_8$?

It is known that the Kitaev Hamiltonian and its spin-liquid ground state both break the $SU(2)$ spin-rotation symmetry. So what's the spin-rotation-symmetry group for the Kitaev model? It's obvious ...
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29 views

1D Hubbard model in the negative U limit

In the 1D Hubbard model at half-filling, is the ground state considered as a charge-density wave (CDW) state in the very negative U limit? Is there a long range order exist in this case? Is a CDW ...
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0answers
20 views

Snowmaking in the tropics - an estimate of water evaporation

If I set up a snowmaker in the tropics and sprayed water with it how much water would I evaporate? How would I calculate?
1
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1answer
30 views

Difference between non-collinear systems and paramagnetic ones?

Non-collinear magnetism and paramagnetism, are they the same thing?
2
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1answer
98 views

Functional field integral in condensed matter field theory (Altland)

This is the action for the 1+1 dimensional interacting electron system; $$S_{cl}[\theta , \phi]= \frac{1}{2\pi} \int dxd\tau \left(g^{-1}v(\partial_x \theta)^2 + gv(\partial_x \phi)^2 + ...
4
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1answer
112 views

Anyons only in 2+1 spacetime dimensions - better explanation

Regrading why anyons exist only in 2+1 spacetime dimensions (which have an arbitrary phase on exchange), I read the reason that the paths for exchange in 3D are deformable into each other while in ...
2
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1answer
66 views

How to calculate energy in two-band Hubbard model

It might be a very easy question for you, but I am confused and I need helps. In the simplest Hubbard model at one-dimensional lattice, I ignore the $U$ term and only remain the hopping term. ...
2
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1answer
104 views

Why do He-3 atoms repel each other much more strongly than electrons?

Is there a simple answer to this question ? see last line of this paragraph http://en.wikipedia.org/wiki/Fermionic_condensate#Fermionic_superfluids
2
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1answer
81 views

What happens to the free energy of the two-dimensional ising model with vortices?

The classical 2d Ising model has a Hamiltonian of the form: \begin{equation} H = -\sum_{m,n = 0}^{M,N} J_1 x_{m,n}x_{m+1,n} + J_2 x_{m,n}x_{m,n+1} \end{equation} The partition function for the model ...
5
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435 views

Why can, or can not, a perfectly incompressible fluid exist?

Water is normally assumed to be an incompressible fluid - for example in the context of calculations involving water pressure. I wondered whether that is strictly true, or an approximation? Later I ...
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1answer
49 views

'Pseudo-Relativistic' behavior in Graphene

I've read that electrons in Graphene behave 'pseudo-relativistically'; what does this mean? how do they behave differently from electrons in other materials?
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45 views

What is $\epsilon_\infty$ in this equation and why can it be neglected in the IR?

I'm reading this paper (warning, PDF) and they mention that the complex permittivity $\epsilon$ and complex conductivity $\sigma$ are related through the equation $$\epsilon - \epsilon_\infty = (4\pi ...
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61 views

Questions on degenerate ground states and the thermodynamic limit?

For example, let's consider a $N$ spin-1/2 system on a lattice described by the Hamiltonian $H$. My questions are: (1) If $H$ has either global $SU(2)$ spin-rotation symmetry or time-reversal ...