Questions tagged [quantum-spin]

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.

348 questions with no upvoted or accepted answers
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1answer
859 views

Did the Feynman heuristic of “simple effects have simple causes” fail for spin statistics?

Someone here recently noted that "The spin-statistics thing isn't a problem, it is a theorem (a demonstrably valid proposition), and it shouldn't be addressed, it should be understood and celebrated." ...
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1answer
778 views

Sequential Stern-Gerlach devices - realizable experiment or teaching aid?

At least one textbook [1] uses sequential Stern-Gerlach devices to introduce to students that the components of angular momentum are incompatible observables. Viz., the $z$-up beam from a SG device ...
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1answer
807 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 ...
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690 views

Chirality and helicity operators for the massless bispinor rep and their generalisation on arbitrary (tensor, 4-vector etc) cases

Let's have chirality projection operator $$ \hat {C}_{\pm} = \frac{1 \pm \gamma^{5}}{2}. $$ We introduce it and called it chirality, because $$ \hat {C}_{+}\psi = \begin{pmatrix} \psi_{\alpha} \\ 0 \...
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2k views

What is “spin-orbit torque?”

I am trying really hard to understand the concept of spin-orbit torque. It is a new-ish discovery in the field of spintronics and has many applications for magnetic devices. The information that has ...
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2answers
344 views

Spin drift velocity?

I am currently reading this Phys Rev paper by H C Torrey. In this paper, he derives the Bloch equations with an additional diffusion term. He says that the current density is given by $$\mathbf j_{\...
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516 views

Why the spin 3/2 particle equation would violate causality?

I've recently come around the study of the so called Rarita-Schwinger equation for elementary particles of spin $3/2$. The point it the article is really short, and no book treats the topic in a very ...
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200 views

Free Will Theorem question

The Kochen-Specker Theorem says, if I understand it correctly, that the results of spin measurements cannot be predetermined independent of measurement. They get to this conclusion by describing 33 ...
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460 views

Chirality, helicity and their relationship for the massless case

Chirality can be interpreted as a property of Lorentz group - Lorentz transformation of field through representation $(s, 0)$ or representation $(0, s)$. For the massless particles one says, that ...
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56 views

Gilbert original paper (1955) reference

I'm looking for the Gilbert's original paper where he derives the gyromagnetic Landau Lifshitz Gilbert (LLG) equation of motion from a variational principle. Most of the people cite: Gilbert, ...
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111 views

About the rigour of replacing spins by hardcore Bosons

In literature one sometimes find that spins are replaced by hardcore bosons. Formally one replaces spin operators $\sigma^- \leftrightarrow a$, $\sigma^+ \leftrightarrow a^\dagger$, $\sigma_z \...
5
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1answer
141 views

Direct measurement of $\mathbf S^2$

For concreteness, consider two spin-1/2 particles with spin vectors $\mathbf S_i=(S_i^x,S_i^y, S_i^z)$. The total Hilbertspace is four-dimensional, and as a basis we can take the (simultaneous) ...
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331 views

What type of fields are continuous spin representations?

Continuous spin representations (infinite dimensional representations of the Lorentz group) are pretty rarely discussed, and usually not in that much mathematical details. And usually it is done in a ...
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136 views

Why is a particles magnetic moment proportional to its spin?

the magnetic moment of a particles is given by, m=kS, where k is a constant the gyromagnetic ratio but where does this equation come from, is it just from experiments?
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45 views

How does one electron or spin see the other in a ferromagnetic material?

QM tells us that only one component of angular momentum is measurable which conventionally taken to be Lz= 1/2. The other two components have an uncertain magnitude and direction and this is usually ...
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113 views

Physical/geometrical interpretations of spinors?

Physically, a scalar is a quantity invariant with reference frame, a vector is a quantity associated with a direction, tensors are higher relationships between vectors - what are spinors? I thought I ...
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375 views

Confused by Malus Law

The well known Malus law predicts $\cos^2\theta$ for the probability of passing through a filter oriented with an angle $\theta$ w.r.t. the polarization direction of the incident photon. On the other ...
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109 views

Mathematical Rationale for Fermion and Boson Spin Representations

I am beginning with the statement that: All physical states occur as one dimensional representations of $\mathfrak{S}_n$; they are either bosonic or fermionic. Where a fermionic state of n identical ...
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218 views

Does the spin1/2 rotation operator rotate spin in real space?

in quantum mechanics, the rotation operator for spin one half is $R_{\alpha}\left(\hat{\boldsymbol{n}}\right)=\mbox{exp}\left(-i\frac{\alpha}{2}\boldsymbol{\sigma}\cdot\hat{\boldsymbol{n}}\right)$. ...
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119 views

Interpretation of inequivalent spin structures (on the circle)

I was wondering about the physical interpretation of inequivalent spin structures on a given configuration space. For simplicity, I'd be satisified by only discussing the case of the circle. There ...
4
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1answer
84 views

What physical principle determines microstates in quantum mechanics?

If we want to calculate mean magnetisation of an equilibrium two-level-system, we know that we can resolve the identity $ \mathbf{1} = \sum_i | E_i \rangle \langle E_i |$ and giving us a uniform ...
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108 views

A question about the emergence of 'spin' from relativistic QM

I know that quantum-spin is not equivalent to the spinning of a classical object about an axis passing through it, although there are some similarities. I also know that spin naturally emerges out of ...
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91 views

Anyon Lagrangian

The discussion of 2D quantum mechanics of two identical particles may be started (as written in the following article) with Lagrangian $$ L_s = \frac{\theta}{\pi} \dot{\phi} $$ where $\phi$ is ...
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94 views

How long does it take to a local perturbation to propagate along a quantum system?

Imagine to have a one-dimensional system in its ground state, and to apply a local perturbation at one edge of the system. How does the system evolve after being perturbed? More specifically, how ...
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193 views

Spin via Change of Phase

Thinking of spin as arising from a change in the phase of a wave function: The angular momentum is defined by the change of the phase of the wave function under rotations, which may come from the ...
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114 views

How to prove that identical particles are attracted or repelled in a given spin-s interaction theory?

Let's assume that we have integer spin interaction theory (EM field, linearized gravity, arbitrary gauge spin s theory). How to prove the consequence that in interaction theory with spin $s = 2n$ two ...
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267 views

Dirac equation in curved spacetime - found second derivatives of the metric, violation of the principle of equivalence?

I am working on the Dirac equation on curved spacetime. A Foldy-Wouthuysen transformation was applied to obtain the semiclassical limit of the equation to study the dynamics of the spin of the ...
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158 views

What's the difference between exchange spin wave and magnetostatic spin wave?

So far I've heard of three kinds of spin waves Magnetostatic spin waves (MSW) Dipole-exchange spin waves (DESW) Exchange spin waves (ESW) What's the difference?
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75 views

Why can a renormalizable quantum field theory only include spin 0, 1/2 and 1 fields?

Hitoshi Murayama writes in his 221A Lecture Notes on Spin How do we choose spin when you introduce a field, then? A consistent ( i.e. , renormalizable) quantum field theory can include only spin 0, ...
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1answer
188 views

Second order relativistic corrections to Pauli equation from Dirac equation

I'm trying to derive the full and correct Hamiltonian for spin$\frac{1}{2}$ particles from Dirac equation up to second order in $v/c$. For a potential and magnetic field constant in time. In ...
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54 views

Spin squeezing in quantum mechanics

The definition of spin squeezing in quantum mechanics is given as follows: Spin squeezing is a technique used to surpass the Standard Quantum Limit (SQL) of uncertainty in measurement (which for ...
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37 views

I can't find the Einstein relation for spin diffusivity

I'm studying the article "Thermodynamic analysis of interfacial transport and of the thermomagnetoelectric system" (PhysRevB.35.4959, Mark Johnson and R. H. Silsbee); they use this relation $$ D=\frac{...
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1answer
106 views

What is the 4-spin vector of a photon?

The photon, being a vector boson has 2 spin states, $\pm 1$. In relativity, we can determine the four-spin vector $s^{\mu}$ of a particle (see e.g. Costa et al. 2017). What would $s^{\mu}$ be for a ...
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1answer
44 views

Weyssenhoff fluid and Frenkel condition

A Weyssenhoff fluid is a continuos fluid with spin. The spin is described by an antisimmetric tensor $s{_{ab}}=s{_{[ab]}}$ satisfying the Frenkel condition \begin{equation} s{_{ab}}u{^b}=0 \end{...
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63 views

How similar are the spin states and the matter/antimatter states?

Within the Dirac formalism, we have bispinors that represent both if a particle is spin up or spin down, and if a particle is an electron or a positron. And these representations are very similar. (...
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570 views

A question in Quantum Phase Transition of Transverse Ising Model

In section 1.4 quantum Ising model of Subir Sachdev's book Quantum Phase Transitions, he discusses the quantum phase transition of transverse quantum Ising Model at zero temperature (so we just focus ...
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350 views

What is the relation between the Holstein-Primakoff Transformation and Bethe's Ansatz for the Heisenberg Ferromagnet?

Bethe's Ansatz is a method to find the eigenenergies and eigenstates of the Heisenberg ferromagnet (see also spin waves). For a general n-excitation state it involves solving rather complicated ...
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117 views

Does this physical situation distinguish whether you are viewing it a mirror?

The weak interaction's lack of $P$-symmetry is often explained by saying that "the amplitudes for processes involving the weak interaction are different from the amplitudes for the same processes ...
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49 views

How does the electromagnetic field of an electron and a rotating ball of charge behave in a co-rotating reference frame?

First time poster, hope I'm not breaking any rules. Basically I'm curious about how far the classical analogy of an electron as a rotating ball of charge can be stretched. The situation I'm ...
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97 views

How is Spin experimentally determined?

I know what spin is and how theories determine it for particles. What I don't understand yet, is how people, through experiments and data analysis or whatever, reach to confirm/say that the $X$ ...
3
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1answer
105 views

In what circumstances, can exchange interaction acquire temperature dependence?

Heisenberg exchange interaction (sometimes called as magnetic stiffness?), originating from the Coulomb interaction and the Fermion statistics, is widely used in theories of magnetism. Conventionally, ...
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549 views

Transfer from Heisenberg to Ising model

It is well know, that ferromagnets can be described using Hamiltonian $$ H = -\sum\limits_{i<j}J_{ij}\, (\mathbf{s}_i \cdot \mathbf{s}_j). $$ where (three dimensional) spins $\mathbf{s}_i$ ...
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512 views

The Heisenberg Uncertainty in Bose Einstein condensates

What happens to the Heisenberg uncertainty principle, when a system reaches the Bose-Einstein condensed state? In our statistical mechanics lecture, we derived the following formula for the fraction ...
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80 views

Would CP Violation happen in a universe with four spatial dimensions?

The weak force is the force that violates CP symmetry as the way it effects a particle depends on that particles handedness. Particles have a property known as spin although it is different from ...
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0answers
63 views

Spin of an operator in supersymmetric theories

How exactly is the spin of an operator in the context of a supersymmetric theory defined? For example, in page 25 of [1], $\mathcal{N} = 2$ supersymmetry is defined to have operators $J, G^{+}, G^{-}, ...
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568 views

Attraction and repulsion of electron spin ups and electron spin downs

Alright, we know that copper is a diamagnetic material, which has paired electrons. These paired electrons have different spin. I'm specifically interested in what is going on with the electrons in a ...
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218 views

What are the assumptions behind “term symbols”?

In multi-electron atoms, the electronic state of the optically active "subshell" is often expressed in "term symbols" notation. I.e. $^{2S+1}L_J$. This presumes that the system of electrons has ...
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356 views

Reducing massive representation of the Poincare group to the massless one

I want to ask about the connection for massive and massless representation of the Poincare group. Sorry for the awkwardness. First I must to represent the formalism for both of cases. Massive ...
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0answers
130 views

Demo of Classical Example of a Spin 1/2 System

I was wondering if there are demos of spin 1/2 systems that anyone knows of. Are there any classical systems that are symmetric under a rotation of $4\pi$ and not $2\pi$?
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892 views

The connection between classical and quantum spins

I have two questions, which are connected with each other. The first question. In a classical relativistic (SRT) case for one particle can be defined (in a reason of "antisymmetric" nature of ...