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Physical Review Letters

Coherent Spin Manipulation and ESR on Superfluid Helium Nanodroplets

Markus Koch, Gerald Auböck, Carlo Callegari, Wolfgang Ernst

Published Jul 15, 2009• 32 citations
Publisher

Abstract

Superfluid helium nanodroplets provide a versatile substrate to cool atoms and molecules, and to assemble weakly bound complexes. Absence of spin-relaxation mechanisms makes He nanodroplets ideal to isolate open-shell atoms and create a spin-polarized system. Here, we show the first coherent manipulation of such a system by resonant excitation of a magnetic-dipole transition. Observation of approximately 50 Rabi oscillations demonstrates coherent population transfer with minimal dephasing. Ours is also the first application of ESR spectroscopy to doped He nanodroplets. This unique environment results in extremely sharp lines and hyperfine-resolved spectra: those of single 39K and 85Rb dopant atoms presented here denote an increase of the Fermi contact interaction, which we can follow as a function of droplet size, reflecting the distortion of the valence-electron wave function due to the surrounding He.

DOI
10.1103/physrevlett.103.035302

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Superfluid helium nanodroplets provide a versatile substrate to cool atoms and molecules, and to assemble weakly bound complexes. Absence of spin-relaxation mechanisms makes He nanodroplets ideal to isolate open-shell atoms and create a spin-polarized system. Here, we show the first coherent manipulation of such a system by resonant excitation of a magnetic-dipole transition. Observation of approximately 50 Rabi oscillations demonstrates coherent population transfer with minimal dephasing. Ours is also the first application of ESR spectroscopy to doped He nanodroplets. This unique environment results in extremely sharp lines and hyperfine-resolved spectra: those of single 39K and 85Rb dopant atoms presented here denote an increase of the Fermi contact interaction, which we can follow as a function of droplet size, reflecting the distortion of the valence-electron wave function due to the surrounding He.
Superfluid helium nanodroplets are an ideal environment for the formation of metastable, self-organized dopant nanostructures. However, the presence of vortices often hinders their formation. Here, we demonstrate the generation of vortex-free helium nanodroplets and explore the size range in which they can be produced. From x-ray diffraction images of xenon-doped droplets, we identify that single compact structures, assigned to vortex-free aggregation, prevail up to 10^{8} atoms per droplet. This finding builds the basis for exploring the assembly of far-from-equilibrium nanostructures at low temperatures.
Superfluid helium nanodroplets are an ideal environment for the formation of metastable, self-organized dopant nanostructures. However, the presence of vortices often hinders their formation. Here, we demonstrate the generation of vortex-free helium nanodroplets and explore the size range in which they can be produced. From x-ray diffraction images of xenon-doped droplets, we identify that single compact structures, assigned to vortex-free aggregation, prevail up to 10810^8108 atoms per droplet. This finding builds the basis for exploring the assembly of far-from-equilibrium nanostructures at low temperatures.
Superfluid helium droplets represent a unique state of matter, which are large clusters of helium typically containing approximately 103-1011 helium atoms and exhibit remarkable properties such as superfluidity, a very low temperature (0.37 K) and high thermal conductivity. This PhD project investigates two novel aspects of superfluid helium droplets: the use of superfluid helium nanodroplets as the nanoreactors to grow magnetic nanoparticles and the generation and exploration of quantum vortices in a controlled manner. In the first part, we exploit the very low temperature and rapid cooling to develop a new approach for fabricating magnetic nanoparticles. For the very low temperature and the ultrahigh thermal conductivity, superfluid helium can suppress thermal effects during the atom-by-atom growth of magnetic nanoparticles, making the relatively weak exchange interactions (compared with metallic bonding) the driving force. As a result, the atomic spins align for ferromagnetic elements and thus the magnetic moments of nanoparticles are maximized. In particular, we focus on iron nanoparticles coated by a gold shell and investigate their properties by electron microscopy (for structural investigation) and x-ray circular dichroism (XMCD), for magnetic property measurement) at the Diamond Light Source. We first study mass spectrometry of small iron clusters and observed abnormal behaviours. Unlike other molecular clusters formed in helium droplets such as water, gold and silver, which typically follow a Poisson distribution, Fe+ channel was found to be far greater than that of FeN+ (N = 2-8) clusters. We postulate this as an indicator for the formation of high-spin iron clusters inside superfluid helium and attempt to provide an interpretation based on DFT calculations. However, XMCD showed an expected low magnetisation for Fe/Au core-shell nanoparticles which is even lower than iron oxide nanoparticles, indicating that the neutral Fe atoms are oxidized into Fe2+ within the nanoparticles which is magnetically inert. This is accounted by the very high electron negativity of Au atoms and the alloying effect during the growth of nanoparticles, which dismisses the magnetic properties. Our work shows that the choice of protective shell is important to maintain the magnetic properties of iron nanoparticles and points the direction for the next-step research. The second part presents a breakthrough in quantum vortex research. We demonstrate a novel method for generating controlled vortex arrays in superfluid helium droplets through collisions with cesium ions. Subsequential addition of metal atoms (Ag and Au) to helium droplets and the formation of nanodroplets allow the vortex lattices to be imaged after the nanoparticles are deposited onto a solid surface. By this approach we have revealed a record-high vortex density of 5.6×10¹⁴ m⁻², exceeding previous observations in bulk superfluid helium by more than six orders of magnitude. This unprecedented vortex density opens new possibilities for studying quantum hydrodynamics at extreme angular momenta and investigating quantum turbulence in previously inaccessible regimes. Through detailed theoretical analysis and experimental characterization, this work establishes superfluid helium droplets as a versatile platform for both materials synthesis and fundamental research. Our findings not only advance the understanding of superfluidity but also provide a new pathway for developing high-performance magnetic nanomaterials that can potentially revolutionize biomedical science and technologies.
Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, exploring the dynamic properties of individual droplets is experimentally challenging. In this work, we used single-shot femtosecond x-ray coherent diffractive imaging to investigate the rotation of single, isolated superfluid helium-4 droplets containing ~10(8) to 10(11) atoms. The formation of quantum vortex lattices inside the droplets is confirmed by observing characteristic Bragg patterns from xenon clusters trapped in the vortex cores. The vortex densities are up to five orders of magnitude larger than those observed in bulk liquid helium. The droplets exhibit large centrifugal deformations but retain axially symmetric shapes at angular velocities well beyond the stability range of viscous classical droplets.
We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{*}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{*}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.
We experimentally demonstrate that the rotation of molecules embedded in helium nanodroplets can be controlled with an optical centrifuge, allowing for the study of molecular dynamics inside the strongly interacting many-body environment of superfluid helium at variable levels of rotational excitation. By doping the droplets with dimers of nitric oxide, (NO)_{2}, and measuring the degree of their centrifuge-induced alignment as a function of time, we show both the forced in-field rotation of molecules in a continuous range of frequencies, as well as the field-free resonant rotation with a long nanosecond-scale decay. The ability to control and monitor the rotational dynamics of molecular rotors inside the superfluid medium may shed new light on superfluidity and the interaction of superfluids with defects at the atomic level.