A Comprehensive Study of A GaAs Spin Polarized Source

William Newman Author

04/04/2021 Added

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An overview of spin electron physics and the generation of spin polarized sources from GaAs shards. Experimental polarization results and description of the system used to measure them.

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[00:00:00.960]Hi, I'm William Newman. And I will be talking about gallium arsenide (GaAs),
[00:00:04.740]and its pivotal role in the production of spin polarized electrons.
[00:00:08.490]This talk is meant for the general public.
[00:00:10.230]So I apologize if any of this information isn't new too you.
[00:00:15.660]First, we will begin talking about electrons,
[00:00:18.360]their property is spin and an electron beam's polarization.
[00:00:22.200]We will then move on to discussing the history of GaAs and its
[00:00:25.590]capabilities as a source of spin,
[00:00:27.840]polarized electrons, our GaAs source that we use for this experiment
[00:00:32.370]and our results and outlook for the future.
[00:00:36.330]So.
[00:00:36.690]Electrons have a property called spin,
[00:00:39.210]the spin of electron points along a direction.
[00:00:42.510]This is where it's often best to visualize the electron has a ball that is spinning.
[00:00:47.460]Despite that analogy being far from reality. Electrons have what we call one
[00:00:52.440]half spin. This this is an important property for any measurement involving
[00:00:57.360]electron spin. Essentially this one half quantity,
[00:01:00.510]specifies that the electron can be measured in one of two states with
[00:01:05.160]spend parallel or anti-parallel to our measurement direction.
[00:01:10.380]This is commonly referred to as up or down respectively.
[00:01:14.160]To elaborate on this,
[00:01:15.450]if you're going to make a measurement of a quantity dependent on the spin
[00:01:18.290]electron, you will only be able to measure that quantity in two states up or down.
[00:01:24.540]We then define the polarization of electrons as the number of electrons with spin
[00:01:28.770]up minus the number of electrons of spin down divided by the sum.
[00:01:33.150]The polarization for one electron is always one or minus one.
[00:01:37.620]For two electrons,
[00:01:38.340]we can measure both spins as spin up and thus our electron polarization
[00:01:43.110]is one. Alternatively, we can measure both electrons having opposite spin.
[00:01:48.000]So our polarization for the two electrons is zero.
[00:01:51.930]This type of measurement is then scaled up for measuring multiple electrons and
[00:01:56.190]then percentage polarizations become more applicable. For example,
[00:01:59.940]if we had a beam of electrons with 30% of them is being spin up and 70% being
[00:02:04.830]spin down, we have a net polarization of 40%.
[00:02:10.950]Now that we've covered the basics of spin and the measurement of polarization,
[00:02:14.820]we move on to GaAs and how we can generate a source of polarized electrons.
[00:02:18.780]Briefly, before we cover GaAs. First,
[00:02:23.070]we need to discuss circularly polarized light, like electrons,
[00:02:26.940]photons have spin.
[00:02:28.080]The light we measure can either be left-handed or right-handed polarized light.
[00:02:32.700]The two animated images you see show how the electric field of light rotates,
[00:02:36.900]depending on the helicity of light.
[00:02:39.120]This helicity is important when describing light's
[00:02:41.670]interaction with materials and electrons in that material.
[00:02:45.750]We can use this light to control how we interact with it and influence the
[00:02:50.400]spin of electrons within the material when they
[00:02:53.100]when they interact with light of a given handedness.
[00:02:57.660]What makes GaAs especially interesting in electron spin physics is that with
[00:03:02.500]circularly polarized light,
[00:03:03.880]you can create a source of electrons where there are three times as many
[00:03:08.320]electrons with one type of spin than the other.
[00:03:12.040]This results in a theoretical maximum of 50%,
[00:03:17.110]the polarization 50%. To achieve this type of emission, however,
[00:03:21.190]is difficult with GaAs. Typically,
[00:03:24.010]what you would have to do is you have to apply layers of cesium and oxygen onto
[00:03:27.970]the surface of GaAs in order for the electrons,
[00:03:30.850]to be able to obtain enough energy to escape out
[00:03:33.430]into vacuum and away from the bulk material.
[00:03:37.390]This process is called activation and it takes quite some time to accomplish.
[00:03:42.370]It also requires a good vacuum system and a clean sample.
[00:03:46.150]The difficulty of this process is what makes it not applicable to all
[00:03:49.780]situations. Electron emission can be achieved easily though,
[00:03:54.520]using nanotips.
[00:03:56.020]So our research group has looked towards creating GaAs nanotips so
[00:04:00.010]that we have a source that can produce electrons easily.
[00:04:03.460]This next section of the talk we'll go over.
[00:04:05.320]What is required to achieve electronic emission from these tips,
[00:04:09.820]images shown here are nano tips made from tungsten,
[00:04:13.180]and these can provide us with the structure to easily emit electrons
[00:04:17.800]sizes ranging from a few hundred to a few thousand atoms.
[00:04:21.880]Normally continuous wave light is not strong enough to be able to emit
[00:04:26.860]electrons. To do that, we must employ pulsed light,
[00:04:31.270]which is very strong, with electric field,
[00:04:33.730]very strong for very short periods of time.
[00:04:37.270]This in combination with the nanometer size structure works symbiotically to
[00:04:41.860]emit electrons strongly.
[00:04:44.830]We try to create similarly small structures with GaAs rather ineloquently.
[00:04:50.200]We carefully and scientifically smash gallium arsenide wafers to create these shards,
[00:04:55.000]which then when looked under an electron microscope reveals that there are small
[00:04:59.170]fine structures that are under the size constraints needed for strong
[00:05:02.890]electron emission.
[00:05:05.860]Now that we have covered the details of what we are trying to measure and the
[00:05:09.730]source we want to achieve our goals with,
[00:05:12.100]we can delve into the experimental setup we have and the results of recent measurements.
[00:05:18.940]With a pump laser, we give enough laser power
[00:05:20.926]to an oscillator which generates pulses of light in
[00:05:23.260]the infrared region.
[00:05:24.790]We then send these pulses of light through polarization optics,
[00:05:28.000]which allows us to control the helicity of light and make them circularly
[00:05:31.030]polarize. We then send this,
[00:05:33.370]this light into the chamber where it's reflected off of a mirror inside that
[00:05:37.690]focuses onto the GaAs.
[00:05:39.940]Electrons emitted are then accelerated towards our polarization
[00:05:44.680]measurement device.
[00:05:46.990]This device consists of a gold rod in the center of which electrons scatter off
[00:05:51.340]this scattering is spin dependent,
[00:05:53.440]which is why we can use this to measure the electron polarization.
[00:05:58.840]We count electrons for both,
[00:06:00.910]left-handed polarized light onto the GaAs,
[00:06:03.550]as well as right-handed polarized light.
[00:06:05.680]This gives us a overall value that we can then use to calculate the polarization
[00:06:10.330]of our electron beam.
[00:06:13.720]Here is a table,
[00:06:14.850]that summarizes polarization data for changes in two system parameters
[00:06:19.690]and a color contour plot to compliment it.
[00:06:22.690]We've been able to achieve high polarization,
[00:06:25.420]which is promising and exciting for our group. However,
[00:06:28.720]we can also achieve polarizations that are larger than what we should
[00:06:31.990]theoretically be able to achieve. Namely,
[00:06:34.750]the one measurement of 63% and the two of above 90%.
[00:06:39.370]Right now, we are looking into these inconsistencies and are resolving them.
[00:06:46.000]For future work, we plan on exploring,
[00:06:49.090]GaAs sources like etched GaAs,
[00:06:52.420]and a GaAs nano tip array that we have created with
[00:06:57.310]a highly polarized source of this type,
[00:06:59.740]there are many experiments that can be conducted from exploring fundamental
[00:07:04.000]principles like Pauli exclusion principle for free electrons.
[00:07:08.440]Learning more about GaAs as a source of electrons.
[00:07:11.920]We can also use these to probe other magnetic or spin dependent interactions
[00:07:16.870]with molecules and bulk materials. For example,
[00:07:20.470]Jefferson laboratories uses a source of spin polarized electrons to be able to
[00:07:24.190]measure the size of a neutron.
[00:07:27.790]Last.
[00:07:28.240]I like to thank you for attending this talk as well as I like to thank my
[00:07:31.240]advisor, Dr. Tim Gay and our post-doc Dr. Sam Kermati
[00:07:34.632]as well as the rest of our group members for providing their
[00:07:38.260]valuable insight and knowledge and overall support,
[00:07:41.620]more information can be found on our website.
[00:07:43.960]And I also like to thank the national science foundation for funding this
[00:07:47.050]project. Thank you.

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A Comprehensive Study of A GaAs Spin Polarized Source

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