Fundamental characteristic property of particles which together with orbital angular momentum acts as the generator of rotations and which doesn't have a classical equivalent but is sometimes compared to and contrasted with classical intrinsic angular momentum.

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Spin of unpaired electrons on an atom

Consider an atom which has many unpaired electrons in its outer shell, like iron. What is the spin of these electrons? Are they all 'up'? I know that since they are unpaired, they are more ...
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How do you rotate spin of an electron?

I can't seem to connect these two stories. Can you please help? I have heard that you have to turn electron by 720 degrees in order to get the same spin state. Has this been seen experimentally? How ...
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49 views

Spin orientations

The concept of spin is purely a quantum mechanical effect, it is restricted to given values in given orientations. If hypothetically spin were a classical concept yet still of a fixed value, would its ...
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257 views

Meaning of spin operator

I am learning about spin in QM and I was wondering if $\langle{\psi}|\hat{S}_z|\psi\rangle$ where $\psi$ is a spin wave function, is a meaningful quantity? In the case of the Hamiltonian $\hat{H}$, ...
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41 views

Connection between half and whole integer eigenvalues for orbital angular momentum [duplicate]

I have been trying to follow this derivation from Sakurai and Shankar, pulling from both. I would like to see how the following derivation can be extended to orbital angular momentum, and thus find ...
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69 views

The uncertainty principle and spin

I realize that this may be a very basic question, but I've been unable to find the answer elsewhere so thanks in advance for the help. Suppose an electron's spin is measured about an axis, and then ...
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QM rotation operator

I have seen the proof that for fermions a rotation of $2 \pi$ does not return a spin angular momentum eigenstate to its original form, but instead multiplies the wavefunction by $-1$. Here is an ...
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153 views

Spin of vector boson in higher dimension

A vector boson $V^{\mu}, \mu = 0,...,3$ has spin 1. To my understanding (correct me if I'm wrong) this is because it transforms as a 4-vector under Lorentz transformation $SO(1,3)$. So the $\mu = ...
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Half-integer spin and infinitesimal rotations

On p. 692 of 'Quantum Mechanics' by Cohen-Tannoudji, he states that: Every finite rotation can be decomposed into an infinite number of infinitesimal rotations, since the angle of rotation can ...
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28 views

Reducing unitary evolution operator of a two-spin system to the evolution operator of one of the spins

Consider a system of two spins $s_1$ and $s_2$, each of which can be in one of two states, represented by 0 or 1. A basis for the Hilbert space of this system would be {|0,0>,|0,1>,|1,0> and |1,1>}, ...
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488 views

Does the spin of electrons imply a more complex structure than has been observed? [duplicate]

If electrons have no substructure and are considered point particles (according to the Standard Model), then how can they also have intrinsic spin? It would seem that the fact that they exhibit spin ...
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256 views

Can spin-1/2 emerge as a property of quasiparticles if original description of the system was without spin?

When we consider a band structure of some crystal, we can get a model of particle-antiparticle system like electrons and holes. In graphene, for instance, we even get a model of massless Dirac ...
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An identity of Pauli matrices

I am studying spin recently, and textbook gives some identities of Pauli matrices, one said that for any two unit vectors $\bf m$ and $\bf n$, $[\bf m \cdot \bf{\sigma},\bf {n \cdot \sigma}]= ...
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representation of spinors

I am trying to get from the abstract representation of Spinors, as wave functions $|\Psi \rangle$ in the base of tensors products $| S_z \rangle \otimes | x \rangle$ of eigenvectors of the spin ...
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28 views

modern exposition of exact ground state of classical XY model or Ising model

What is the state of art technique in solving exact ground state of Heisenberg model, meaning minimization of the H terms (hamiltonian) provided infinite spin space? ...
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97 views

How to associate a Hilbert space with a QM system?

I couldn't really find a fitting title for this question. I'm still relatively new to QM and am trying to get the basics down. I understand that a physical system is associated with a Hilbert Space, ...
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Spin connection and covariant derivative

How to prove explicitly (i.e., to don't postulate it) that by including Lorentz indices $a$ the covariant derivative $D_{\mu}$ looks like $$ D_{\mu}A^{\nu a} = \partial_{\mu}A^{\nu a} + ...
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Relation between two sets of generators of SO(3) [migrated]

I am working with the spin 1 representation of SU(2), which is just SO(3). The ordinary generators used in quantum mechanics are: $J_x = \left( \begin{array}{ccc} 0 & \frac{1}{\sqrt{2}} & 0 ...
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48 views

Can hydrogen atom state be a superposition of 2 pure states with opposite spin?

The task is: We are performing measurements on hydrogen atom, that is in an unknown state $\psi$. $\psi$ is a superposition of $n=1$ and $n=2$ pure states and is orthogonal to ...
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Does point group symmetry also act within “spin space” for a lattice spin system?

As an example, let's consider a quantum spin system on a 2D square lattice. The lattice point group symmetries include $C_4$ rotation, parities, etc.... And let's take $C_2$ symmetry (2-fold rotation) ...
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74 views

Are these two spin states the same?

Consider two sets of axes, $xyz$ and $x'y'z'$, and the two spin states \begin{align} |\psi\rangle &= A(|+_x\rangle + |+_y\rangle + |+_z\rangle)\\ |\psi'\rangle &= A(|+_{x'}\rangle + ...
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75 views

Why do particles have spins such as $1/2$, $3/2$, $5/2$? [duplicate]

What does it mean to have 'half' spin? I have looked on Wikipedia and a few youtube videos on spin but they don't explain what it means to have $1/2$ spin. I am 18 and only starting to learning about ...
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69 views

How can we measure chirality in experiments?

Chirality is a concept quite different from helicity. These two concepts only happen to have the same numerical value for massless particles. I understand that we can measure helicity, but how can we ...
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109 views

How does De Broglie–Bohm theory or pilot wave theory explain the results of the Stern–Gerlach experiment?

The Copehagen interpretation of QM explains the Stern–Gerlach experiment by asserting that a particle is in a superposition of states and doesn't have a definite spin until measured. However, the de ...
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Why does the Stern–Gerlach quantum spin experiment conflict with classical mechanics?

My understanding of the Stern–Gerlach experiment is that neutral (0 total charge) particles are sent through a non-homogeneous magnetic field, with the expectation that the field will push that ...
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21 views

Azimuthal angle in bloch sphere

I was wondering if there is any direct physical meaning behind the Bloch sphere parametrization in the sense that if I choose the angle $\phi =0$, then what does this mean for my spin state? Does ...
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97 views

Goldstone modes of spin density wave

A spin density wave (SDW) is a phase in which a material suddenly shows a periodically modulated spin density $S_{\vec{q}}(\vec{r}) $ below a certain critical tempereature $T_C$. Obviously some kind ...
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72 views

Question about surface term in QFT problem

I am trying to follow the solution of the following problem (Srednicki 39.2): To show that: $$J_z b_s^\dagger(p\hat z)|0\rangle=\frac{1}{2}\ s\ b_s^\dagger(p\hat z)\ |0\rangle, $$ where $J_z$ ...
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118 views

Numerical study of Hubbard Model and Spin Charge Separation effect

Hi, How can i implement the creation operator effect on the ground state(in FORTRAN)? we calculate the ground state using modified Lanczos method,and we obtain a vector(array) with lots of numbers ...
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208 views

Is it proper to describe electron spin as being similar to light polarity?

We always hear that electron spin is merely analagous to angular momentum in classical mechanics, but it never seems to be followed up by what is actually going on based on first principles (not even ...
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45 views

Coherence of spin state

In my textbook the coherence of a spin state $|\psi\rangle $ is measured by the quantity $|\langle \uparrow|\psi| \downarrow \rangle|$. The thing is that I am not sure how this quantity is related to ...
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Basis spin states

We are given a system of $N$ spin states and the following (non-hermitian) Hamiltonian $$H = \frac{N \hbar \nu}{2M} \sin(\alpha)+ \sum_{i=1}^N \frac{\hbar \omega_i }{2} \sigma_{z,i} + \frac{\hbar \nu ...
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112 views

How to interpret spin observables constructed by non-standard phase choices?

If we try to find matrix elements of ladder operators ( $J_{\pm}$) for spin when they act on eigenstates of $J^2$ and $J_z$ ( $\newcommand{ket}[1]{\left|#1\right\rangle} ...
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102 views

Klein Gordon for spin-1 particle photon

If Klein Gordon equation is for spin-0 particles, I write massless fields as $\square A=0$, how can I say $A_\mu=\epsilon^\mu e^{-ikx}$ as a wave function of polarized photon (spin-1) ?
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77 views

Finding the Direction of Measurement Given the Spin State

I am currently trying to gain a fuller understanding of the meaning of various spin states and their relation to the direction of measurement by a Stern-Gerlach device. I came across two spin-${1 ...
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How to explain spin for a 14-year old teen [duplicate]

How do you explain the concept of spin in QM to a 14-year old teen, and why it isn't a vector as in the classical notion of angular momentum?
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75 views

Changing of spin for electron?

Can we change the spin of electron by applying magnetic field from $\uparrow$ to $\downarrow$ configuration?
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117 views

Is there a way to get the spin naturally in nonrelativistic theories?

We all know how spin is added in a rather ad-hoc way in quantum mechanics. In the other hand, in relativistic quantum field theories the spin structure arises quite naturally from the fields. Is ...
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71 views

Why Lagrangian of electromagnetism with Lorenz Gauge evolve Klein Gordon equation?

Simply Lagrangian without a source for Maxwell equation is $$ L = -\frac{1}{4}F^{\mu\nu}F_{\mu\nu} $$ Also Lorenz Gauge condition is $$ \partial_{\mu}A^{\mu}=0 $$ and if so I can briefly add this ...
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28 views

Why does a spinning nucleon generate a magnetic field?

I am trying to understand how NMR works, but I am not sure why a spinning nucleon produces a magnetic field. Is this a consequence of the quark structure inside?
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69 views

Why gamma-matrices are associated with tetrads Lorentz rotation?

In Zee's "QFT in nutshell" in a paragraph "Differential geometry of Riemann manifold" he states that Dirac gamma-matrices are associated with tetrads Lorentz rotation, so Dirac lagrangian in curved ...
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52 views

Interpretations of Stern-Gerlach experiment

In many textbooks (J.J. Sakurai for instance) S-G experiment is explained through the idea of a collapse of a state of a system. So here, in this discrete case, we can see quantum behavior more ...
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75 views

Where do the quantum fields encode the spin information?

I know basically the difference between Klein-Gordon and Dirac field is spin. But I am not sure where we need to implement this info. The solutions of both equations are the wave packets which ...
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271 views

Tensor Product vs. Direct Product for three spin-1/2 particles

Let us consider three spin-1/2 particles and only focusing on their intrinsic spin $S$. The Hilbert space has then to be $\mathcal H = ℂ^2 ⊗ ℂ^2 ⊗ ℂ^2$. The spin can be described by $V ∈ \text{SU(2)}$ ...
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68 views

Spin, parity, etc. conservation in decays/reactions

I have a long list of short physics questions I'm reviewing, and there are several that ask about whether certain decays or reactions are allowed. Clearly these questions are testing my knowledge of ...
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123 views

A conceptual question about spin

I read in Landau's quantum mechanics(section 62) that if the Hamiltonian does not involve spin, the total wave function can be written $$ \psi ( x_1, x_2, ...) \chi ( \sigma_1 , \sigma_2 ,...)$$ in ...
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52 views

Why $2j+1$ number of states?

In this statement from Modern Quantum Mechanics by J.J. Sakurai: If $j$ is an integer, all $m$ values are integers; if $j$ is a half-integer, all $m$ values are half-integers. The allowed ...
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How have the duration of the martian day changed in the past?

There is a panspermia theory which claims that life might have begun on Mars and I currently read a post that the human circadian rhythm is closer to the martian day length (about 25h) than the day ...
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34 views

Electron Operator Trouble in Numerical Calculation of Fermi-Hubbard Hamiltonian

I am trying to build a code in python that calculates the Hubbard Hamiltonian given the basis states. For the two electron (one spin up, one spin down), two site system, I know I have the following ...
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35 views

How to determine whether an eigenstate of total spin is symmetric or antisymmetric?

Here we have two identical paticles with spin $I$, integer or half-integer, and there are $(2I+1)^2$ states. Each one of them can be uniquely determined by total spin and its orientation, we can use ...