research

RESEARCH ACTIVITIES
(names of former students are capitalized)

The primary thrust of the research in my group is to study the influence of high hydrostatic pressures on the magnetic, superconducting and structural properties of exotic condensed matter systems. In this regard the application of pressure is used to test our understanding of interesting physical phenomena as a function of lattice parameter and to search for new phenomena. Some of the samples we study are synthesized in my laboratory. In the following I give a brief summary of the principal research projects of current (and some of past) interest to my group. Click here for PDF files of our publications on these topics since 1999 (plus a few earlier). [Note that 1 GPa = 10 kbar = 10,000 atm = 0.01 Mbar]

PRESSURE-INDUCED SUPERCONDUCTIVITY IN ELEMENTAL YTTERBIUM AND EUROPIUM METALS

Of the 92 naturally occurring elements in the periodic table, 55 are known to be superconducting: 31 at ambient pressure and 24 more only under high pressure. The high-pressure elemental superconductors include such unlikely elements as oxygen, iron, and silicon! The highest known value for an elemental superconductor is T_c = 29 K for Ca. Here is the latest version of the Periodic Table of Superconductivity.

In elemental ytterbium metal the Yb ion prefers to remain divalent to retain its non-magnetic state with 14 4f electrons. Under sufficient pressure one anticipates that an increase in valence would occur whereby one 4f electron jumps into the conduction band, rendering Yb magnetic. In 2018 student JING SONG discovered that instead Yb metal becomes superconducting near 2 K if 86 GPa pressure is applied. X-ray absorption spectroscopy studies in collaboration with former students GILBERTO FABBRIS and WENLI BI together with Dan Haskel show that Yb remains mixed valent to at least 125 GPa (1.25 million atm), pointing to an active role of 4f electrons in the emergence of superconductivity.

Early in 2009 MATHEW DEBESSAI in our group discovered that europium metal becomes superconducting near 2 K for pressures greater than 80 GPa. What makes this discovery particularly interesting is the fact that europium, like almost all lanthanides, possesses a strong local magnetic moment which totally suppresses superconductivity. The fact that europium becomes superconducting under extreme pressures implies that the magnetism has been either destroyed or severely weakened. In fact, if divalent europium becomes trivalent under pressure, as all other rare-earth metals except ytterbium are at ambient pressure, then its ground state would indeed be expected to be non-magnetic. Trivalent europium would exhibit only very weak Van Vleck paramagnetism, like the actinide element americium. Interesting is also that the value of europium's superconducting transition temperature (2 K) is very low compared to that of other trivalent s,p,d-electron elements like Sc, Y, La and Lu (10 - 20 K). X-ray spectroscopy experiments by former graduate students WENLI BI and GILBERTO FABBRIS at the Advanced Photon Source (APS) at the Argonne National Labs revealed that europium remains nearly divalent to pressures of 100 GPa (1 Mbar). In collaboration with Yue Meng, WENLI also carried out extensive x-ray diffraction studies at the APS and discovered four structural phase transitions to 92 GPa in europium metal.

PRESSURE-INDUCED MAGNETIC ORDERING AT ANOMALOUSLY HIGH TEMPERATURES IN LANTHANIDE METALS - GIANT SUPERCONDUCTING PAIR BREAKING IN DILUTE MAGNETIC ALLOYS WITH YTTRIUM

The magnetic state in all elemental lanthanide metals is local-moment in character and relatively stable, with the exception of Ce. Whereas only 0.7 GPa pressure is sufficient to cause the magnetic state of Ce to seriously weaken, much higher (Mbar) pressures are likely necessary to render the magnetic state of the other lanthanides unstable, a state where Kondo physics, heavy Fermion behavior, valence fluctuations followed ultimately by a complete increase in valence, whereby one 4f electron jumps into the conduction band. One exception is Gd where the stability of the half-filled 4f-state is extraordinarily stable, requiring much higher pressure to destabilize.

In 2015 student ISAIAH LIM succeeded in tracking the magnetic ordering temperature T_o of Dy to 160 GPa pressure (1.6 million atmospheres!). T_o follows a very circuitous route, initially decreasing rapidly with pressure before passing through a minimum and raising rapidly above Dy's volume collapse pressure to values well above ambient temperature, by far the highest magnetic ordering temperature of any lanthanide. We suggest that such a strongly enhanced ordering temperature may be yet another, completely unexpected property of the Kondo lattice state. These interesting results under pressure on Dy prompted ISAIAH LIM, JING SONG, and YUHANG DENG to undertake similar studies on Tb, Nd, and Sm. Here also the magnetic ordering temperature rose to anomalously high values.

AC susceptibility measurements by these students on dilute magnetic alloys of Pr, Nd, Sm, Gd, Tb, and Dy with the superconducting host Y revealed giant pair breaking as large as 40 K per atomic percent, a record high value. Such giant pair breaking is only possible if Kondo physics is involved. This means that the magnetic ion is approaching a magnetic instability. Since the giant pair breaking and anomalously high values of the magnetic ordering temperature occur in the same pressure region, the superconductivity may well be caused by magnetic fluctuations.

ANOMALOUS MAGNETISM AND PHASE TRANSITION IN THE SEMI-HEUSLER COMPOUND CuMnSb

Postdoc PALLAVI MALAVI carried out electrical resistivity measurements on CuMnSb under pressure to 53 GPa using the group's diamond anvil cell in collaboration with Alexander Regnat and Andreas Bauer in Prof. Pfleiderer's group in the TU Munich. She observed a sudden disappearance of magnetic ordering above 8 GPa in the temperature range below ambient. X-ray diffraction studies together with former students WENLI BI AND JING SONG at the APS synchrotron revealed a cubic-to-tetragonal phase transition. It appears that the magnetic ordering temperature shifted to temperatures well above ambient, the normal temperature range for magnetic ordering in semi-Heusler compounds.

SEARCH FOR AN INSULATOR-TO-METAL TRANSITION IN BENZENE NEAR 200 GPa

Roald Hoffmann and Neil Ashcroft asked the question: at what pressure does benzene become a metal and possibly superconduct? Their calculations indicated that this should occur near 200 GPa. Student NARELLE HILLIER used beveled diamond anvils with central flats of only 0.1 mm diameter and a rhenium gasket to exert a pressure of 2.1 Mbar (2.1 million atm) on a tiny benzene sample. Unfortunately, to this extreme pressure benzene remained transparent to visible light. It benzene had become metallic, it would have blocked visible light and appeared black.

PRESSURE DEPENDENCE OF SUPERCONDUCTIVITY IN KFe₂As₂ AND LaRu₂P₂

Postdoc NEDA FOROOZANI studied the effect of hydrostatic pressure on T_c for the Fe-based pnictide superconductor KFe₂As₂ to 7.1 GPa pressure using helium as pressure medium. This research was proposed by Valentin Taufour and Dan Canfield at Iowa State University in Ames. In this compound there was considerable disagreement between various groups on how T_c depended on pressure. The pressure dependence T_c(P) appeared to depend sensitively on the quality of the pressure medium used. NEDA's T_c(P) measurements on this compound to 7.1 GPa using the most hydrostatic pressure medium known, helium, clarified the situation by yielding benchmark values of T_c(P) to 7.1 GPa. This work is important both because it yielded the intrinsic dependence of T_c on pressure, allowing comparison with theory, but also because it made abundantly clear that the superconducting state in KFe₂As₂ is extraordinarily sensitive to shear stress, as was suspected in other pnictide superconductors.

Before the discovery in 2008 of the Fe-pnictides with superconducting transition temperatures near 60 K, the compound LaRu₂P₂ possessed the highest value of T_c of any known pnictide superconductor. NEDA found that T_c initially increased rapidly with pressure, but then at 2.1 GPa T_c suddenly disappeared. This disappearance is believed to arise from the formation of a strong covalent P-P bond joining two neighboring RuP layers, thus changing the compound from a 2D layered structure into a 3D metal.

ANOMALOUS PROPERTIES OF ALKALI METALS AT EXTREME PRESSURE

The elements with the smallest known values of T_c < 0.4 mK are Rh and Li. In 2002 two groups reported that Li becomes superconducting near 15 - 20 K under pressures greater than 20 GPa, confirming an earlier (1986) indication of superconductivity in Li under pressure by Lin and Dunn. Since none of these three experiments used any pressure medium, the hard diamond anvils and stiff gasket walls pressed directly onto the Li sample, generating shear stresses and plastic deformation. The question is whether the superconducting state is intrinsic or perhaps only arises because of the shear stresses. We decided to use the softest solid known, dense helium, as pressure medium; surrounding the very reactive Li sample with helium might also have a further bonus -- the reduction of reactions between Li and diamond. On her very first try, student SHANTI DEEMYAD loaded Li into a rhenium-gasketed diamond-anvil cell along with tiny ruby spheres and helium pressure medium and reached a pressure of 67 GPa, a record pressure for our group at that time! She indeed confirmed superconductivity in Li above 20 GPa, but the detailed dependence of T_c on pressure, reaching values as high as 14 K, differed considerably from that published earlier. The large increase in T_c for pressures between 20 and 30 GPa is highly unusual and, according to work by Jeffrey Neaton and Neil Ashcroft in 1999, due to the fact that the very large compression of Li brings the ion cores close enough together to force the conduction electrons into interstitial sites. Strong anomalies in all electronic, magnetic, and lattice properties are the result. The strong enhancement in the pseudopotential not only pushes T_chigher, but also can lead to symmetry-breaking phase transitions, induced magnetic ordering, and many other anomalous properties. This ground breaking work pointed the way to many further experiments on the alkali metals under high pressure conditions. For example, Takahiro Matsuoka in Katsuya Shimizu's group has shown that above 70 GPa Li actually turns into a semiconductor and, above 110 GPa, to a superconducting metal again.

The only other alkali metal besides Li known to superconduct is Cs near 2 K under 11 GPa pressure, reported by J. Wittig more than 40 years ago. In 2019 student YUHANG DENG discovered superconductivity in elemental Rb metal near 2 K for pressures above 55 GPa. Rb thus became the 55 elemental solid to become superconducting under ambient or high pressure. See Periodic Table of Superconductivity. Superconductivity in Cs and Rb appear for a pressure at a tetragonal-to-orthorhombic structural phase transition. Since this transition also occurs near 1 Mbar for the alkali metal K (potassium), superconductivity would appear likely near this pressure.

Following theoretical work by Feng, Ashcroft, and Hoffmann at Cornell University, we studied CaLi₂ under extremely high pressures and confirmed that this compound also exhibits the anomalous properties found for elemental Li and Ca. This implies that the predictions of Ashcroft's group, that all properties (electronic, structural, magnetic) become highly anomalous under pressures sufficient to bring the ion cores in close proximity, is not restricted to elemental solids but has very general validity for all forms of matter such as multielement compounds and alloys, crystalline or amorphous. This is now one of the principal thrusts of our group. Many exciting experiments remain to be carried out.

ENHANCEMENT OF SUPERCONDUCTIVITY IN Sc, Y, La, Lu TO MEGABAR PRESSURES

Students JAMES HAMLIN and MATHIEW DEBESSAI studied the d-electron superconductors Sc, Y, and Lu to pressures of almost 2 Megabars and found that T_c reaches values as high as 20 K (Y and Sc), the second highest value for any elemental superconductor! All three of these elements are not known to superconduct at ambient pressure. As with the alkali and alkaline-earth systems above, this anomalous behavior of T_c originates from the sharp reduction in the space available to the conduction electrons as extreme pressure pushes the ion cores together. A very interesting systematics reveals itself for Sc, Y, Lu, and La if their T_c's are plotted versus the relative amount of free volume available to the conduction electrons outside the ion cores.

More generally, these interesting phenomena are but a precursor of what happens to all properties of solids if astronomic pressures are applied which are sufficient to actually break up the shell structure of the constituent atoms, leaving only a Thomas-Fermi gas behind.

RESEARCH ACTIVITIES (names of former students are capitalized)