
Negative Resistance with a Single Atom - manojr
http://sciencebulletin.org/archives/8902.html
======
conistonwater
I liked this explanation better, it's rather clearer:
[https://physics.aps.org/articles/v9/155](https://physics.aps.org/articles/v9/155)

Here is the original report of the effect:
[https://journals.aps.org/pr/abstract/10.1103/PhysRev.109.603](https://journals.aps.org/pr/abstract/10.1103/PhysRev.109.603)

~~~
pizza
That explanation really reminded me a lot of the Petkau effect [0]:

> _The Petkau effect is an early counterexample to linear-effect assumptions
> usually made about radiation exposure._

> _Petkau had been measuring, in the usual way, the radiation dose that would
> rupture a simulated artificial cell membrane. He found that 3500 rads
> delivered in 2 1⁄4 hours (26 rad /min = 15.5 Sv/h) would do it. Then, almost
> by chance, Petkau repeated the experiment with much weaker radiation and
> found that 0.7 rad delivered in 11 1⁄2 hours (1 millirad/min = 0.61 mSv/h)
> also ruptured the membrane. This was counter to the prevailing assumption of
> a linear relationship between total dose or dose rate and the consequences._

[0]
[https://en.wikipedia.org/wiki/Petkau_effect](https://en.wikipedia.org/wiki/Petkau_effect)

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jerf
I offer free useless internet points to anyone who can explain to the next
level of detail what this might improve about current electronics, and a bit
more about the "how".

~~~
jeffwass
In a much older role (nearly 20 yrs ago) my team tried using (quite
successfully) a Resonant Tunneling Diode (RTD) as an electronic pulse
generator to drive a fiber mode-locked laser.

Fun note - the pulses it produced were so narrow that one of the visiting
oscilloscope dealers (I forget if it was LeCroy or Tektronix) was using it to
test out timing resolution of their top model oscilloscopes! (Normally we had
to use a sampling scope to see the pulses).

It was a "quantum well", and IIRC grown as using layers of doped GaAs. (I
might be way off here, I didn't work on the fabrication).

If you plot current vs voltage across a device : an ideal textbook resistor
has a straight line. Normal resistors will have some curvature but generally
be monotonically increasing.

Out RTD showed a current that went up, then down a bit, then back up again. We
biased it with a chosen current and load that put the current smack in the
negative differential region. If you draw a horizontal line through this
current, it intersects the graph at three voltage points. The middle voltage
point has a negative slope, and is an unstable state. At this bias level, the
device would flip back and forth between the two same-current points on either
side where the slope was positive.

The cool part of this thing was that each 'bounce' sent an EM wave through a
waveguide we attached, like an antenna, and the returning wave would trigger
the next flip. So the length of the waveguide set the period of the
oscillation. It had horrible timing jitter on its own, but we could make it
_very_ stable by driving it with a sinusoid oscillator.

~~~
jeffwass
Following on my comment, back around 2004 or so when I was doing my PhD in
physics, I made a Java applet showing how resonant tunnelling works (ie, the
phenomenon that drives the negative differential resistance in the RTD above),
and also how band structure arises in an ordered crystal.

I put these on GitHub, if anyone wants to play with it, this applet is from
the top screenshot :
[https://github.com/jeffwass/Physics_Simulations/blob/master/...](https://github.com/jeffwass/Physics_Simulations/blob/master/README.md)

Basically, in one dimension, we represented an atom as an infinitely-thin
'spike' in potential, a Dirac Delta function.

For a single 'atom', regardless of how much energy an incoming electron wave
function has, some energy is always reflected back and a smaller amount goes
through. (Here, energy and frequency are basically synonymous, where E = h *
nu, h is Planck constant and nu is the frequency.)

For two atoms (a double barrier), certain input frequencies cause destructive
interference of the reflected quantum wave function at the first atom (ie -
the reflections bouncing off the first and second atom cancel out), and
_exactly_ 100% of the incoming energy transfers through! At certain other
frequencies, the output waves cancel and the wave is entirely reflected. The
plot of transmission vs input frequency looks sinusoidal, between 0% and 100%.
The applet shows the impact of adjusting the spacing and relative 'height' of
the barrier atoms.

This interference is the quantum mechanical basis of resonant tunnelling.

In the applet, you can control how many atoms are in the crystal. As you add
more atoms to the system, the output transfer function starts forming notable
regions of high transmission and regions of low transmission.

This is for a perfectly ordered crystal (same potential and spacing). The
applet also shows what happens if you randomise the spacing or the potential
heights.

Question for the HN community : as Java web applets are seemingly obsolete
these days, any recommendations on current front-end tech for making these
types of simulations?

~~~
pizza
three.js! + some light gui toolkit

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tdeck
Those of you who're interested in obscure engineering history might like to
read up on Oleg Losev[1] who developed and built solid-state negative
resistance radios and amplifiers long before the first practical transistors.

[1]
[https://en.wikipedia.org/wiki/Oleg_Losev?wprov=sfla1](https://en.wikipedia.org/wiki/Oleg_Losev?wprov=sfla1)

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tehwalrus
Full text of the original article,

[https://arxiv.org/abs/1608.06344](https://arxiv.org/abs/1608.06344)

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jpmattia
I know the HN protocol is to use the headline, but the sciencebulletin article
headline is pretty broken. Negative differential resistance (NDR) in tunneling
diodes has been understood for several decades, and is pretty far from a
mystery.

The original article is about an I-V curve from a structure involving a
single-atom + a scanning-tunneling microscope (STM). That such a system would
also exhibit NDR is not particularly surprising. Tunneling to a structure with
discrete energy levels will have current flow when energies line up, and less
current flow when energies don't line up. So even the "mystery" isn't much of
a mystery.

The interesting results imho are:

1\. The team got reproducible single-atom tunneling with an STM tip, and

2\. They measured the time-response of about 10 microseconds.

fwiw.

Edit: Resonant tunneling diodes used to be a hobby of mine:
[https://dspace.mit.edu/handle/1721.1/38419](https://dspace.mit.edu/handle/1721.1/38419)

~~~
justifier
it reads like there is a bit more 'mystery', or 'the yet explained',
explained(o):

> measuring current vs voltage in a new way, by applying brief voltage pulses
> to the STM tip.
    
    
          When the pulses lasted 10 microseconds or longer, 
        they saw NDR. 
          But when they reduced the pulse length to 10
        nanoseconds, the effect disappeared. 
        ...
          Combining this information with other data, 
        the team determined the timescale for electrons
        refilling the empty lower level.
    

unreal

(o)
[https://physics.aps.org/articles/v9/155](https://physics.aps.org/articles/v9/155)

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pmalynin
I'm glad to see my University up there. As Canadians we are often left out of
the global scene when it comes to news and publications.

