Electron spin coherence in metallofullerenes: Y, Sc, andLa@C82

Abstract

Endohedral fullerenes encapsulating a spin-active atom or ion within a carbon cage offer a route to self-assembled arrays such as spin chains. In the case of metallofullerenes the charge transfer between the atom and the fullerene cage has been thought to limit the electron spin phase coherence time (T2) to the order of a few microseconds. We study electron spin relaxation in several species of metallofullerene as a function of temperature and solvent environment, yielding a maximum T2 in deuterated o-terphenyl greater than 200µs for Y, Sc and La@C82. The mechanisms governing relaxation (T1, T2) arise from metal-cage vibrational modes, spin-orbit coupling and the nuclear spin environment. The T2 times are over 2 orders of magnitude longer than previously reported and consequently make metallofullerenes of interest in areas such as spin-labelling, spintronics and quantum computing.The possibility of encapsulating an atom inside a fullerene cage was discovered by Heath et al. [1] in 1985 and has led to widespread research into these novel materials. Encapsulation of a nitrogen atom within a C 60 cage (N@C 60 ) has been the subject of particular interest due to the remarkably long electron spin coherence times (T 2 ) reported from 80 to 250 µs [2,3]. Metallofullerenes, containing metal ions encased in a similar way, benefit from faster purification and higher production yields than N@C 60 . However, they have not shown particularly long coherence times (< 1.5 µs), attributed to much greater spin density on the fullerene cage [4,5].The prospect of exploiting the self-assembly of spinactive fullerene molecules into larger structures [6] has stimulated interest in spintronics [7] and quantum information processing (QIP) [8,9]. In particular, metallofullerenes can self-assemble within carbon nanotubes in a 'peapod' structure, creating a 1-D spin chain [6,[10][11][12]. For the potential of such structures to be fully realised, longer electron spin coherence times are required of the constituent metallofullerenes.Several electron paramagnetic resonance (EPR) studies have been conducted on the metallofullerenes Sc-, Y-and La@C 82 focusing on geometric and electronic properties of the molecules [4,5,[13][14][15][16][17][18]. These, along with x-ray diffraction measurements, have shown the metal atom to be off centre in the cage [19][20][21], with charge transfer to the cage dependent on the metal ion species [22]. These previous EPR studies have primarily used CW spectroscopy and few T 2 times have have been accurately extracted. Using pulsed EPR, Knorr et al. report a T 2 of 600 ns for Sc@C 82 (in trichlorobenzene solvent at 2.5 K) but a measured T 2 of 4.1 µs in a Y@C 82 sample was attributed to a 'background' signal [4]. Okabe et al. report a temperature and m I dependence of T 1 and T 2 for La@C 82 (in CS 2 ) arising from motional effects of anisotropic interactions and coherence times < 1.5 µs, in the range 183-283 K [5].In this letter we report pulsed EPR studies of spin relaxation times over a range of temperatures ...

The role of the encapsulated atom in the vibrational spectrа of La@C60–Lu@C60 lanthanide endofullerenes

“…In metal endofullerenes (e.g., Y@C 82 , Sc@C 82 , La@C 82 ), spin coherence times can reach 200 ms. These spin-relaxation times are determined by the frequencies of the coupled vibrations of the metal atom and the carbon cage (i.e., metal-cage modes) [5,6].…”

Hybrid Solid-State Qubits: The Powerful Role of Electron Spins

“…Such endohedral fullerenes 1 based on group III ions such as Sc-, Y-and La@C 82 possess T 2 times in excess of 200 µs under optimised conditions (37). In the case of the remarkable N@C 60 molecule, atomic nitrogen occupies a high-symmetry position at the centre of the cage, leading to an S = 3/2 electron spin with the longest coherence times of any molecular electron spin: 80 µs at room temperature rising to 500 µs at temperatures below 100 K (38, 39).…”

Spin Quantum Computing with Endohedral Fullerenes

“…The related systems of metallo-fullerenes 4 and other molecular atom cages like silsesquioxanes 5 are largely disregarded here, although recent studies indicate that they may also have some favorable properties for quantum information science. [6][7][8][9] After a brief introduction to solid-state spin quantum computing ( §1), we start with fundamental considerations about the system architecture for a scalable fullerene-based quantum register element ( §2), outline the main developments in endohedral fullerene materials science relevant for realizing such a register ( §3), review experimental implementations of quantum operations in endohedral fullerene ensembles ( §4), illustrate the ongoing quest to realize an efficient single-spin read-out for these materials ( §5), and conclude with a very brief outlook on further experimental challenges and opportunities ( §6).…”