High-Sensitivity Nuclear Magnetic Resonance at Giga-Pascal Pressures: A New Tool for Probing Electronic and Chemical Properties of Condensed Matter under Extreme Conditions

Summary

Nuclear magnetic resonance is one of the most important spectroscopic tools. Here, the development of a new approach under high pressure, currently up to 10.1 GPa, is presented. This opens a new window into condensed matter physics and chemistry, where high-pressure research is of great importance.

Abstract

Nuclear Magnetic Resonance (NMR) is one of the most important techniques for the study of condensed matter systems, their chemical structure, and their electronic properties. The application of high pressure enables one to synthesize new materials, but the response of known materials to high pressure is a very useful tool for studying their electronic structure and developing theories. For example, high-pressure synthesis might be at the origin of life; and understanding the behavior of small molecules under extreme pressure will tell us more about fundamental processes in our universe. It is no wonder that there has always been great interest in having NMR available at high pressures. Unfortunately, the desired pressures are often well into the Giga-Pascal (GPa) range and require special anvil cell devices where only very small, secluded volumes are available. This has restricted the use of NMR almost entirely in the past, and only recently, a new approach to high-sensitivity GPa NMR, which has a resonating micro-coil inside the sample chamber, was put forward. This approach enables us to achieve high sensitivity with experiments that bring the power of NMR to Giga-Pascal pressure condensed matter research. First applications, the detection of a topological electronic transition in ordinary aluminum metal and the closing of the pseudo-gap in high-temperature superconductivity, show the power of such an approach. Meanwhile, the range of achievable pressures was increased tremendously with a new generation of anvil cells (up to 10.1 GPa), that fit standard-bore NMR magnets. This approach might become a new, important tool for the investigation of many condensed matter systems, in chemistry, geochemistry, and in physics, since we can now watch structural changes with the eyes of a very versatile probe.

Introduction

Since Percy Bridgman's hallmark experiments of condensed matter under high hydrostatic pressures at the beginning of the last century, the field of high pressure physics has evolved rapidly¹. A large number of intriguing phenomena are known to occur under pressures of several GPa². In addition, the response of condensed matter systems to high pressure has taught us a lot about their electronic ground and excited states^3,4.

Unfortunately, techniques for the investigation of the electronic properties of condensed matter at Giga-Pascal pressures are rare, with x-ray or DC resistance measurements leading the way⁵. In particular, the detection of electronic or nuclear magnetic moments with electron spin (ESR) or nuclear magnetic resonance (NMR) experiments, is bound to be almost impossible to implement in a typical high-pressure anvil cells where one needs to retrieve the signal from a tiny volume enshrined by anvils and a sealing gasket.

Several groups have tried to solve this problem by using complex arrangements, e.g., two split-pair radio-frequency (RF) coils wound along the flanks of the anvils⁶; a single or double loop hair-pin resonator^7,8; or even a split rhenium gasket as a RF pick-up coil⁹, see Figure 1. Unfortunately, those approaches still suffered from a low signal-to-noise ratio (SNR), limiting the experimental applications to large-γ nuclei such as ¹H¹⁰. The interested reader may be referred to other high-pressure resonant tank circuit experiments^11–15. Pravica and Silvera¹⁶ report the highest pressure achieved in an anvil cell for NMR with 12.8 GPa, who studied the ortho-para conversion of hydrogen.

With great interest in applying NMR to study the properties of quantum solids, our group was interested in having NMR available at high pressures, as well. Finally, in 2009 it could be demonstrated that high-sensitivity anvil cell NMR is indeed possible if a resonating radio-frequency (RF) micro-coil is placed directly in the high-pressure cavity enclosing the sample¹⁷. In such an approach, the NMR sensitivity is improved by several orders of magnitude (mostly due to the dramatic increase in filling factor of the RF coil), which made even more challenging NMR experiments possible, e.g.,¹⁷O NMR on powder samples of a high-temperature superconductor at up to 7 GPa¹⁸. Superconductivity in these materials can be greatly amplified by the application of pressure, and it is now possible to follow this process with a local electronic probe that promises fundamental insight into the governing processes. Another example for the power of NMR under high pressure emerged from what were believed to be routine referencing experiments: in order to test the introduced new anvil cell NMR, one of the best known materials was measured – simple aluminum metal. As the pressure was increased, an unexpected deviation of the NMR shift from what one would expect for a free-electron system was found. Repeated experiments, also under increased pressures, showed that the new results were indeed reliable. Finally, with band structure calculations it was then found that the results are the manifestation of a topological transition of the Fermi surface of aluminum, which could not be detected by calculations years ago, when the computing power was low. Extrapolation of the findings to ambient conditions showed that the properties of this metal that is used almost everywhere are influenced by this special electronic condition.

In order to pursue a number of different applications specially designed anvil cells (previous cells had been imported from the Cavendish Laboratory and retrofitted for NMR) have been developed. Currently, the used home-built chassis are capable of reaching pressures up to 25 GPa using a pair of 800 µm culet 6H-SiC anvils. NMR experiments were successfully conducted up to 10.1 GPa, so far. The NMR performance of this new cells was shown to be excellent¹⁹. The main component is Titanium-Aluminum(6)-Vanadium(4) with an extra low interstitial level (grade 23), providing a yield strength of about 800 MPa²⁰. Due to its non-magnetic properties (the magnetic susceptibility χ is about 5 ppm) it is an adequate material for the anvil cell chassis. The overall dimensions of the introduced cells (see Figure 2 for an overview of all home-built anvil cell designs) are small enough to fit into regular standard bore NMR magnets. The smallest design, the LAC-TM1, which is only 20 mm in height and 17 mm in diameter, fits also typical small, cold-bore magnets (30 mm bore diameter). The LAC-TM2, which is the latest chassis the authors designed, uses four M4 Allen countersink bolts (made out of the same alloy as the cell chassis) as pressure driving mechanism, allowing for a smooth control of the internal pressure (blue prints attached in supplementary section).

Typically, diamond anvils are used in order to generate highest pressures of above 100 GPa. Xu and Mao^21–23 have demonstrated that moissanite anvils provide a cost effective alternative in high-pressure research, up to pressures of about 60 GPa. Therefore, moissanite anvils were used for the introduced GPa NMR approach. The best results were achieved with customized large-cone 6H-SiC anvils from the anvil department of Charles & Colvard. With those cells, for pressures up to 10.1 GPa, the use of 800 µm culet anvils was found to result in very good NMR sensitivity. For comparison, Lee et al. report a SNR of 1 for ¹H NMR of tap water, while the SNR of the introduced micro-coil approach showed a value of 25 for 1/7 of their volume, even at a somewhat lower magnetic field.

With this new approach to high-sensitivity anvil cell NMR one can pursue many applications that promise exciting new insight into the physics and chemistry of modern materials. However, as always, sensitivity and resolution ultimately limit the application of NMR, in particular, if one is interested in much higher pressures that demand smaller culet sizes. Then, one has not only to optimize the cell design with even smaller RF coils, but also think about methods for increasing nuclear polarization.

Protocol

1. Mounting and Aligning of the 6H-SiC Large Cone Boehler-type Anvils

1. Fix the piston and x-y plate in the mounting tools and insert the Boehler-type anvils in the seating area.
2. Make sure each anvil sits firmly in the backing plates.
3. Using epoxy resin, (e.g., Stycast 1266), glue both anvils to their seats. Cure for 12 h at RT, or 65 ºC in a furnace for 2 hr.
4. For a sufficient anvil alignment, use the M1 set-screws to align the backing plates and monitor the parallelism of both anvils. If the anvils were found to be non-parallel, remove the epoxy resin and restart at point 1.2.

2. Gasket Preparation

1. Drill 1 mm holes into a chip of annealed Cu-Be (Cu 98 w%, Be 2 wt%, thickness of 0.5 mm) for the brass guide pins.
2. Insert three 5 mm long pieces of 1 mm diameter non-insulated copper wire into the holes, which are distributed along the anvil, to serve as guide pins for the Cu-Be gasket.
3. Check for proper grounding between the guide pins and the cell body. Typically, a DC resistance of about 0.1 Ω is desired. Improve with an application of a small amount of conductive silver.
4. Place the Cu-Be chip on top of the moissanite anvil and close the cell.
5. Using a hydraulic press, pressurize the gasket to about 1/8^th of the culet diameter for maximized working stability. Monitor the actual thickness of the indentation using a micrometer caliper.
6. Drill a hole of the appropriate diameter (½ of the culet diameter) in the center of the indentation.
7. Carve two channels into the pre-indented gasket. The channels should be deep enough to accommodate the 18 µm copper wire of the micro-coil.
8. Harden the prepared gasket at 617 K for 2 to 3 hr in a furnace.

3. Preparing and Loading of the Micro-coil

1. Use a piece of 1 mm copper wire and thread it through the feed-through of the piston. Fix the copper wire with epoxy resin and cure it according to step 1.3.
2. Choose an awl (see list of materials) which has the desired diameter for the micro-coil and fix it between a pair of rotatable chuck-jaws.
3. Glue (with e.g., varnish from SCB, see list of materials) one end of the 18 µm copper wire onto the chuck jaws, while holding the other end and rotate the chuck jaw so that the wire is coiled onto the awl.
4. When the micro-coil is of the desired geometry, fix the other end of the wire onto the glue as well.
5. Use diluted varnish to fix the coil by applying a small amount on top of the windings.
6. Remove the coil carefully from the awl using Teflon tape.
7. Place some epoxy resin (see point 1.3), without any additives, in the channels of the gasket.
8. Place the micro-coil inside the sample chamber and fix the leads into the channels.
9. Cure the epoxy resin according to step 1.3.
10. Solder one lead of the micro-coil to the hot wire and the other to a guide pin.
11. Add some silver conductive paste on top of each junction. Curing typically takes some minutes.
12. Seal both junctions with a small amount of epoxy resin.
13. Cure the epoxy according to step 1.3.
14. Now, check the DC resistance of the coil after every step.
15. Place the sample in the micro-coil. Be aware that any unnecessary physical contact may destroy the coil.
16. Add finely ground ruby powder to the sample for pressure calibration.
17. Finally, flood the sample chamber with an appropriate pressure medium. Use paraffin oil to ensure nearly-hydrostatic conditions up to 9 GPa.
18. Close the cell carefully.

4. Applying and Monitoring Pressure

1. At first, slightly tighten the M3 Allen countersunk screws.
2. For pressurization fix the cell in a vise. Now, tighten two opposing screws pairwise.
3. Place the pressurized cell in an appropriate cell holder.
4. Adjust the position of the cell so that the laser beam reaches the sample chamber.
5. Use the fine-adjustment table to focus the ruby powder in the laser beam.
6. Monitor the ruby photoluminescence spectrum using the corresponding spectrometer software.
7. Extract the actual pressure in the sample cavity from the observed spectral shift of the ruby R1 and R2 lines.
8. Equilibrate the pressurized cell for at least 12 hr before NMR measurements are started.

5. Performing NMR Experiments

1. Mount the pressure cell onto a typical NMR probe. Manufacture appropriate cell holders in a mechanical workshop.
2. Solder the hot wire to the probe. Check for proper electrical contact between the cell and the probe.
3. Now, perform standard NMR experiments. Draw attention to the fact that the micro-coil is very sensitive to the applied radio-frequency power.

Results

Figure 3 shows how the completely assembled pressure cell, the wiring, and the mounting onto a typical NMR probe look like. In the following, several experiments will be reviewed which should enable the reader to gather a broad overview about the benefits and limits of the introduced technique.

Discussion

A new and promising method to perform NMR at Giga-Pascal pressures was described. This method opens up the door to a broad variety of NMR experiments due to its excellent sensitivity and resolution. Nevertheless, several steps described in the protocol section are crucial to the outcome of the experiment. Especially, the preparation of the micro-coil and its fixation in the Cu-Be gasket is very difficult and requires some experience. In the following, some important tips are given, which should help a first successful ap...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Titanium grade 23	robemetall GmbH	ASTM F 136
Beryllium copper foil	GoodFellow	CU070501	Alloy 25 (C17200)
Copper wire for micro-coil	Polyfil		quote on inquiry
Stycast 1266	Sil-Mid Ldt.	S1266001KG
Moissanite anvils	Charles & Colvard		quote on inquiry
Paraffin oil (pressure medium)	Sigma Aldrich	18512-1L
M4 Allen contersunk screws (Ti64)	Der Schraubenladen	DIN912 M4x20
Optiprexx PLS	Almax-easylab		quote on inquiry
Ruby spheres (~10-50 µm)	DiamondAnvils.com	P00996
Manual Toggle Press	DiamondAnvils.com	A87000
Gasket Thickness Micrometer	DiamondAnvils.com	A86000
Titanium Scalpel	Newmatic Medical	NM45200710421
Glass-writing Diamond	Plano	54467
Smoothing Awls	Flume	1 4444 001
Chuck-jaws (4 jaws)	Flume	4 561 289
Lathe	Flume	4 560 023
Drilling Machine	Flume	4 570 020
Drill chuck	Flume	4 570 021
XY stage	Flume	4 570 022
Drills (0.30 to 0.50 mm)	Flume	4 572 652 – 654
Low Temperature Varnish	SCBshop	SCBltv03