All Questions
Tagged with spin-models solid-state-physics
19
questions
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Spin polarization due to exchange? Would spin polarized ground state exist if no $e$-$e$ repulsion?
Non-relativistic no-magnetic-field many electron hamiltonian contains no spin operators. How would spin polarization happen in many electron ground state (modeled by LSDA DFT for instance)?
I often ...
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Explanation of massive Goldstone modes
I'm solving this exercise with a Heisenberg Hamiltonean in linear spin-wave theory and at some point we are asked to compute the dispersion relation at $k=0$, which leads me to finding two different ...
- 548
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Bragg-Williams microcanonical esemble
In this question Bragg-Williams theory of phase transition of the forum someone was asking for Bragg-Williams aprox. and how to calculate entropy. The answer is clear and correct, the Bragg-Williams ...
- 123
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Polaron transformation in quantum optics
I'm trying to understand the so-called polaron transformation as frequently encountered in quantum optics. Take the following paper as example: "Quantum dot cavity-QED in the presence of strong ...
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What is helical Dirac nature?
A concept in Spintronics which can not be found on Wikipedia. The picture is from a review of Spintronics of 2016 by Fert.
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Why is the gauge group of pseudo-fermion mapping referred to as $\mathrm{SU}(2)$ and not $\mathrm U(2)$?
The representation of spin $\frac{1}{2}$ operators $\hat{S}^{a}$ by pseudo-fermions (also called Abrikosov fermions) is defined by the mapping
$$
\hat{S}^{a} = \frac{1}{2} \text{Tr}\big[ \hat{\psi}^{\...
- 11
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Is it possible to construct an operator for $z$-component of spin for a 2D system?
Let's say we have an arrangement of spins in 2D space (as given in the below picture).
Assume that the $z$-axis is out of the plane and a spin (circled in red) makes an angle $\theta$ with the $x$-...
- 1,335
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$\mathbb{Z}_2$ gauge theory and disorder
I am confused about basics of $\mathbb{Z}_2$ (and likely other) gauge theories and plain disorder. Let
$$H=H_F + h\,H_{EM}$$
$$H_F = -t\sum_l (c^\dagger_l \sigma^z_{l,l+1} c_{l+1} + h.c.)$$
be (the '...
- 101
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Exchange stiffness for HCP
I am studying the exchange interaction, which can be described with the Heisenberg Hamiltonian:
$\hat{H} = -\sum_{i,j}J_{ij}\hat{\mathbf{S_i}}\cdot \hat{\mathbf{S_j}}$
In the framework of constant ...
- 31
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Analytic methods for studying thin films (approximately 2D spin systems)
I am working on a project involving thin films of a magnetic material represented by essentially an Ising model on a 3 dimensional lattice. The bulk case is relatively straight forward, the ...
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Antiferromagnetic-like interaction, but preferring 120/240 angles instead of 180?
Antiferromagnetic interaction energetically prefers opposite spins for neighbors: rotated by 180 degrees.
But imagine there is $S_3$ symmetry: 3 possibilities rotated by 120 degrees and ...
- 1,010
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Quenched systems - disorder average (SYK model)
In a system with quenched disorder one is usually looking for self-averaging quantities, i.e., quantities such that the average over the couplings produces a ``typical" configuration in the ...
- 327
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136
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The Ising approximation - what exactly is it?
I am slightly confused about the nature of the Ising model to study ferromagnetism. Consider the Heisenberg Hamiltonian with Zeeman term:
\[H=-\frac{1}{2} \sum_{i\ne j}J_{ij} S_i\cdot S_j+g\mu_B {B}\...
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Validity of Ising model for mean field thoery
The Heisenberg model for the Hamiltonian of a ferromagnet is given by:
$$H=-\frac{J}{2} \sum \vec{S}_i\cdot \vec{S}_j+\mu_B B \sum_i S^z_i$$
when performing mean field theory, to find $\chi$, we ...
3
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Fermi "surface" at finite temperature and its measurement in the lab
As we increase the temperature, we know the sharp Fermi surface at zero temperature becomes smeared out at finite temperature $T>0$. (Just think of the Fermi-Dirac distribution, there will be no ...
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