High Pressure Physics

The field of high pressure physics started to intensify in the first part of the last century. P.W. Bridgman at Harvard developed many of the techniques that broadly developed the field, for which he was awarded the Nobel Prize.

Today, there are two methods of studying materials under pressure: static and dynamic high pressure. In static pressures, the sample is under pressure for extended periods of time, and can be either cooled or heated in an oven or laser heated. Although earlier work was carried out with large presses, some of them generating enormous loads to produce high pressures on large samples, the modern trend has been to use diamond anvil cells (DACs). In this case the samples are quite small, of order a few to a few hundred microns thick and of order 20 to 500 microns in diameter. Smaller samples go to higher pressures. The DAC is easy to set up and use by a single investigator and has optical windows to the sample—the diamonds. Pressures as high as 5 megabar have been achieved. The dynamic technique creates a shock wave by impacting a sample with a high velocity projectile and measuring properties in the shock wave that is created. Pressures in the megabar range can be produced and temperatures as high as 10 to 20,000 K. A few smaller or medium sized shock guns exist in universities, but most are at the National Laboratories, Livermore, Los Alamos, and Sandia. We currently do not use this technique at Harvard.

In our research program we study a number of materials. We have in the past studied metllized xenon and hydrogen iodide. We use a variety of techniques, mostly those used on larger macroscopic sized samples, but in this case adopted to the samples in DACs. One of our focuses is hydrogen and the possibility of metallization. This is discussed in more detail, below.
 

Hydrogen under High Pressure

Background: One of the important challenges of this century has been to transform hydrogen to the metallic state by compressing it to very high densities. This problem was first set forth by Wigner and Huntington in 1935 who proposed that at a sufficiently high density the molecules at the lattice sites would dissociate to form an atomic lattice that would be a metal. In more than half a century since, the predicted pressure for metallization has risen from 0.25 megabar to the 3-6 megabar (300-600 GPa) region as experimentalists have probed to ever higher pressures and theorists have refined their calculational techniques. The prediction in 1968 by Ashcroft that metallic hydrogen might be a high-temperature, perhaps room temperature superconductor, spurred on the experimental developments to produce this material. Subsequently, it was realized by Ramaker, Kumar, and Harris that even in the molecular state, hydrogen might become metallic by a band overlap mechanism. In recent years this mechanism has been pursued by a large number of researchers and the prediction of the metallization pressure in this form has been as low as 150 GPa. Recent calculations predict that molecular metallic hydrogen might have a substantially higher critical temperature for superconductivity than does the atomic form. Measurements on physical properties of hydrogen have been carried out to pressures as high as 251 GPa in our laboratory; and hydrogen has been pressurized to 342 GPa, but the samples remain transparent and there is as yet no indication of metallization. A recent article from Loubeyre an coworkers reports hydrogen turning black at about 320 GPa so there appears to be some contradiction. Measurements of the equation of state of molecular hydrogen to pressures somewhat over a megabar have been used to calculate the Gibb's free energy. This can then be extrapolated and compared to theoretical equations of state for the metallic phase. When these curves cross, a transition takes place from one phase to the other. The prediction here is around 600 GPa. The current status is that theory has not been good enough to accurately predict any of the metallization transitions. Experiment has gradually raised the bar, aiming at a level which seems to rise higher and higher as the bar goes up. It seems that the bar and the level will eventually cross.

What are we doing? We are developing techniques to push the pressure of hydrogen and its isotopes (deuterium and HD) to the 4 megabar range using diamond anvil cells (DACs). A picture of one of our several DACs is shown below. (figure of DAC) This DAC is made of Vascomax, a hard steel that can be subjected to low temperatures and maintains good dimensional stability and strength. Another DAC, not shown, is made of nonmagnetic beryllium-copper and is used for NMR studies at high pressure. A pressure of millions of atmospheres can be applied to a sample by the simple finger-turning of a screw. More recently, we have been developing the technique of pulsed laser heating of samples, using a pulse of order 100 ns that can easily melt metals, etc. We plan to use this to study metastable states at high pressure.

Our research has always featured developing an understanding of the physics of hydrogen as the pressure bar has been raised. We have observed a number of phase transitions in high pressure hydrogen and its isotopes and studied these systems by optical means such as Raman scattering, infrared absorption, and optical absorption. Below we show the phase diagram that we determined for deuterium; a similar one exists for hydrogen, while HD has some unusual features.

Figure. A DAC used for megabar research. The diamond anvils are in the lower part of the DAC; The cable bundle is for measuring temperature, load, heating, etc. The DAC is about the size of a coke-bottle; it is long and narrow so that it easily fits into a helium cryostat with optical windows.
 

The equation of state of hydrogen and its dielectric properties have been studied using optical techniques. We have developed a new method for performing NMR on samples in a diamond anvil cell. Our technique increases the signal-to-noise by almost a factor of 100 (M.G. Pravica and I.F. Silvera, Rev. Sci. Inst. 69, 479, 1998) and has been recently used to study ortho-para conversion of hydrogen as a function of pressure (preprint available).

Our lab is equipped with optical spectrometers, lasers, ccd detectors, diamond anvil cells of our own design, diamond polishing apparatus, an electric discharge machine (see room 123 photo), microscopy, cryostats, etc.; our latest piece of equipment is a state-of-the-art FTIR spectrometer.

We have a long-standing interest in the ruby under pressure and the ruby pressure scale. We have published a number of papers on the fluorescence spectra of ruby under pressure, and plan to do some new studies at a synchrotron in the near future.

We are developing methods for depositing hard electrodes on diamonds using clean room techniques developed for nano-physics experiments. Why? The most rigorous method of showing that a material is a metal is the simple (outside of a DAC) method of measuring the dc electrical conductivity and showing that it remains finite in the limit that temperature goes to zero. This is a challenge in a DAC as the electrodes must lead from the high pressure environment of the sample to the ambient pressure of the laboratory. It is even more challenging for pressures above 2 megabar due to the small size of the sample. Tune in to follow our progress.



Figure. The phase diagram of deuterium to almost 2 megabars. LP is the low pressure phase known to be hexagonal close packed. The BSP is the broken symmetry phase, in which a quantum transition takes place at about 28 Gpa—at T=0K, the ground state spontaneously distorts from a symmetric to an orientationally ordered phase. The D-A is the mysterious high pressure phase, suspected to be molecular metallic, and having once been claimed to be so by the Geophysical Laboratory Group. In recent years we have shown that there is no experimental evidence that this is metallic, so that the question remains open and the nature of the A-phase is still unknown.