High-pressure superconducting phase diagram of ⁶ Li: Isotope effects in dense lithium

Abstract

We measured the superconducting transition temperature of 6 Li between 16 and 26 GPa, and report the lightest system to exhibit superconductivity to date. The superconducting phase diagram of 6 Li is compared with that of 7 Li through simultaneous measurement in a diamond anvil cell (DAC). Below 21 GPa, Li exhibits a direct (the superconducting coefficient, α, T c ∝ M −α , is positive), but unusually large isotope effect, whereas between 21 and 26 GPa, lithium shows an inverse superconducting isotope effect. The unusual dependence of the superconducting phase diagram of lithium on its atomic mass opens up the question of whether the lattice quantum dynamic effects dominate the low-temperature properties of dense lithium.L ight elements (low Z) and their compounds have been the subject of many recent studies for their potential as hightemperature superconductors (e.g., refs. 1-5). Due to their low mass, the physical properties of the low-Z compounds can be strongly influenced by zero-point effects (lattice quantum dynamics) (6), and mass-related isotope effects may be present in their thermodynamics of vibrational degrees of freedom. Such effects will influence the superconducting properties of these materials. Dependence of superconductivity on isotopic variations of low-Z compounds can be used to probe and determine the magnitude of mass-related effects. This in turn allows better development of models to determine their superconducting properties.Under ambient pressure, lithium is the lightest elemental metallic and superconducting system, and it exhibits one of the highest superconducting transition temperatures of any elemental superconductor under compression (7-11). Despite the large mass difference between the stable isotopes of lithium (∼15%), isotope effects in superconductivity of lithium have not been studied before.In systems with long-range attractive potential, the ratio of lattice zero-point displacements to interatomic distances may increase under compression (increase to the Lindemann ratio at high densities), provided they retain their long-range interactions (12,13). (This is opposed to systems with short-range interactions, e.g., helium, in which the lattice becomes more classical under compression.) In these systems, more deviations from the static lattice behavior are expected at higher densities. At sufficiently low temperatures, where thermal energy is small, lattice quantum dynamics can play a more dominant role in the bulk properties. Sound velocity measurements on stable isotopes of lithium at 77 K and up to 1.6 GPa show that quantum solid effects in lithium, at least in the pressure range studied, do not decrease as a function of pressure (14). Raman spectroscopy studies between 40 and 123 GPa and at 177 K report a reduced isotope effect in high-frequency vibrational modes of Li, which may be related to quantum solid behavior (15). Up to this point, no experiments have reported a comparison of any physical properties of lithium isotopes at low temperatures and high pressures concurren...

Isotope effect in superconducting n-doped SrTiO3

“…In some other rare cases α < 0 has been observed: the pure uranium (α = −2.2) [31], the high-T c superconductor Bi 2 Sr 2 Ca 2 Cu 3 O 10 (α = −0.1) [32], and the metal hydride PdH(D) x [33,34] (−0.3 < α < −0.1). Controversial sign changes of the isotope coefficient have been observed in (Ba,K)Fe 2 As 2 (α = −0.2) [35,36] (due to differences in the sample composition), and in pure lithium under high pressure (α changes with increasing pressure [37]). We will not dwell on the physical origins of the isotope effect in these cases, which are certainly different in the case of uranium, and possibly different in the other examples as well.…”

Dynamical stability of face centered cubic lithium at 25 GPa

“…Thus, it seems that anharmonicity is dynamically stabilizing this system. Plus, this strong phonon renormalization could potentially cause the inverse isotope effect [10] by making the electron-phonon coupling constant differ for the two stable lithium isotopes considering that in PdH anharmonicity explains the inverse isotope effect [25]. Anyway, our frozen-phonon calculations neglect the interaction of the analyzed vibrational mode with the rest of the modes in the crystal.…”

“…For instance, lithium becomes a semiconductor near 80 GPa [8], it melts below ambient temperature (190 K) at around 50 GPa [2] and displays a periodic undamped plasmon according to theoretical calculations [9]. Moreover, it presents one of the highest superconducting critical temperatures (Tc) for an element, reaching values as high as 15 K at around 30 GPa [10][11][12][13]. Recently, the measurement of an inverse isotope effect in the 20-26 GPa range [10] has put this element back under the spotlight.…”

“…Moreover, it presents one of the highest superconducting critical temperatures (Tc) for an element, reaching values as high as 15 K at around 30 GPa [10][11][12][13]. Recently, the measurement of an inverse isotope effect in the 20-26 GPa range [10] has put this element back under the spotlight.…”

Predicted novel insulating electride compound between alkali metals lithium and sodium under high pressure

“…At ambient conditions, all the alkali metals crystallize in the bcc structure [1,2] and display a free-electron like metallic character [3,4] . Application of pressure on these systems results in more complex structures [5,6] and remarkable physical phenomena such as unusual melting behavior [7,8] , Fermi-surface nesting [9] , phonon instabilities [10] , and superconductivities [11][12][13][14] , and transformations into poor metals or even insulators [15][16][17][18][19] . At ambient conditions, the ionic radius of Li has large disparity with respect to the other alkali metals [20] .…”