Quantum spin dynamics

Abstract:

Resonant optical excitation of certain molecular vibrations in κ−(BEDT−TTF)2Cu[N(CN)2]Br has been shown to induce transient superconductinglike optical properties at temperatures far above equilibrium Tc. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator κ−(BEDT−TTF)2Cu[N(CN)2]Cl and the metallic compound κ−(BEDT−TTF)2Cu(NCS)2. We find nonequilibrium photoinduced superconductivity only in κ−(BEDT−TTF)2Cu[N(CN)2]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are prerequisites for this effect.

Abstract:

Magnetite (Fe₃O₄) is of fundamental importance for the Verwey transition near T_V = 125 K, below which a complex lattice distortion and electron orders occur. The Verwey transition is suppressed by chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe₃O₄ nanoparticles leads to an intriguing variation of the Verwey transition: an initial drop of T_V to a minimum at 70 K after 75 days and a followed recovery to 95 K after 160 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution between inhomogeneous to homogeneous doping regimes. This work demonstrates that slow oxidation of nanoparticles can give exquisite control and separation of homogeneous and inhomogeneous doping effects on the Verwey transition and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials.

Abstract:

Electrical control of spins at the nanoscale offers significant architectural advantages in spintronics, because electric fields can be confined over shorter length scales than magnetic fields1,2,3,4,5. Thus, recent demonstrations of electric-field sensitivities in molecular spin materials6,7,8 are tantalizing, raising the viability of the quantum analogues of macroscopic magneto-electric devices9,10,11,12,13,14,15. However, the electric-field sensitivities reported so far are rather weak, prompting the question of how to design molecules with stronger spin–electric couplings. Here we show that one path is to identify an energy scale in the spin spectrum that is associated with a structural degree of freedom with a substantial electrical polarizability. We study an example of a molecular nanomagnet in which a small structural distortion establishes clock transitions (that is, transitions whose energy is to first order independent of the magnetic field) in the spin spectrum; the fact that this distortion is associated with an electric dipole allows us to control the clock-transition energy to an unprecedented degree. We demonstrate coherent electrical control of the quantum spin state and exploit it to independently manipulate the two magnetically identical but inversion-related molecules in the unit cell of the crystal. Our findings pave the way for the use of molecular spins in quantum technologies and spintronics.