By uhoh


2017-01-28 13:05:02 8 Comments

My google news page has burst forth the news that metallic hydrogen has been produced in a laboratory using a diamond anvil cell. Only one service mentions (at this particular moment) that it has been called into question at - at least - premature. That can be read about in Nature: Physicists doubt bold report of metallic hydrogen

Here I only want to ask about the evidence. Is it only the sudden change in visual appearance change as recorded in a camera on top of the microscope, at a specific temperature and pressure? That it "looks shiny?" Or is there actually more direct data than that?

note: I am not asking if the data is sufficient or not, nor if it is convincing, or metallic or not. Just a narrowly defined question about the extent of the observations that have lead to the proposal that a metallic state was achieved.

enter image description here

above: potentially three states of hydrogen as viewed in a diamond anvil cell, from here.

2 comments

@mmesser314 2017-01-28 13:17:43

Too long for a comment.

Abstract from https://arxiv.org/abs/1610.01634

We have studied solid hydrogen under pressure at low temperatures. With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa. We have measured the reflectance as a function of wavelength in the visible spectrum finding values as high as 0.90 from the metallic hydrogen. We have fit the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T= 5.5 K, with a corresponding electron carrier density of $6.7\times10^{23}$ particles/cm$^3$, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.

Edit: As requested, adding Figure 3 from the paper.

enter image description here

Fig. 3. The energy dependence of the normal incidence reflectance off of SMH and the rhenium gasket (P=495 GPa) at liquid nitrogen and liquid helium temperatures. We also show the reflectance from a surface of Re at a pressure of 1 bar at room temperature. The reflectances have been corrected for absorption in the diamond. Filled points are raw data and hollow ones are corrected. The uncertainties in the data points are from measurement of the reflectance and the correction procedure and represent random errors. The lines through the SMH data points are fits with a Drude free electron model; the lines through the Re data points are guides to the eye.

@uhoh 2017-01-28 13:20:08

Great! Is it possible to put a screen capture of Fig. 3 into your "comment" - because it looks suspiciously like a full fledged acceptable answer to me. (Fig. 2 is optional - it seems to be the same as the one in Nature).

@pr1268 2017-01-29 11:07:42

Just curious, what, specifically, is "reflectance" in the context of Fig. 3 above? Light reflectance? If so, then is this a true measure of being "metallic"? (Yes, I do know that high light reflection is indeed a property of a "metal"... but I used to think [still do?] that a metal was defined more by its ability to conduct electricity.)

@uhoh 2017-01-30 02:36:06

@pr1268 if you read the paper first you'll see there is no statement like that at all. The transition of a particular sample of hydrogen from a low reflectance to high reflectance state at a certain pressure is suggestive of a metallic state. Also, metals are high reflectivity as a direct result of their high density of conduction electrons.

@hpekristiansen 2017-02-06 00:10:15

In this youtube video: youtu.be/kV3FY4q1xjY?t=5m1s by SciShow, it is claimed that hydrogen will stay solid at std. pressure. It seems impossible to me. Do you know anything about it?

@akhmeteli 2017-01-28 13:39:56

Judging by https://arxiv.org/abs/1610.01634, the photograph from the microscope is not exactly the only evidence, they also performed reflectance measurement using a spectrometer (see their Figure S5). These two kinds of evidence are closely related, but not identical.

@uhoh 2017-01-28 15:27:33

I agree - but are you adding something that isn't in the other answer already?

@akhmeteli 2017-01-28 15:43:34

@uhoh : I guess the other answer does not respond directly to your question: " Is it only the sudden change in visual appearance change as recorded in a camera on top of the microscope...?"

@uhoh 2017-01-28 16:00:58

I see - so it's actually Figure 3 that has the evidence, or the data, beyond the white light images. Fig. S5 shows a rough sketch of the set-up, but the data is in Fig. 3. The problem is that this kind of measurement, an absolute reflectivity of 0.8 is really hard to do correctly with such a complicated sample geometry and limited access. I think the data are only the points at the three laser wavelengths - the solid lines are calculated. There's no data to back up that they are measuring correctly.

@akhmeteli 2017-01-28 16:23:22

@uhoh : I mentioned Fig. S5 as it demonstrates what and how was measured (not the results, which are indeed in Fig. 3). I neither agree nor disagree that these measurements are problematic, but yours is a "narrowly defined question about the extent of the observations that have lead to the proposal that a metallic state was achieved":-)

@uhoh 2017-01-28 16:34:26

I used to do this kind of optical measurement on micron sized samples, just not in a diamond anvil cell. It's quit a challenge to do it like this, without calibration and without a full spectrum, there are many things that could lead to erroneous results. For what it's worth, I've used the words "data" and "evidence" twice each in the question, but not "how" or "with what".

@akhmeteli 2017-01-28 16:51:47

@uhoh : Again, I did not question your statement on these measurements being problematic (I just cannot tell one way or another). And you did use the wording "as recorded in a camera on top of the microscope", so I believed that "with what" was relevant. Apparently, I was wrong.

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