Quantifying the interplay between fine structure and geometry of an individual molecule on a surface

Abstract

The pathway toward the tailored synthesis of materials starts with precise characterization of the conformational properties and dynamics of individual molecules. Electron spin resonance based scanning tunneling microscopy can potentially address molecular structure with unprecedented resolution. Here, we determine the fine structure and geometry of an individual TiH molecule, utilizing a combination of a newly developed mK ESR-STM in a vector magnetic field and ab initio approaches. We demonstrate a strikingly large anisotropy of the g-tensor unusual for a spin doublet ground state, resulting from a non-trivial orbital angular momentum. We quantify the relationship between the resultant fine structure, hindered rotational modes, and orbital excitations. Our model system provides new avenues to determine the structure and dynamics of individual molecules with unprecedented precision. Main textPrecisely determining the fine structure, dynamics, and geometry of an individual molecule, with sub-molecular resolution, is a grand challenge in numerous fields of nanoscience.Scanning probe microscopy (SPM) has emerged as a surface imaging approach capable of intramolecular resolution of individual molecules [1, 2], quantifying conformational modifications like the static Jahn-Teller distortion [3], or light-assisted conformational changes [4]. Complementary to imaging, SPM-based inelastic excitation spectroscopy (ISTS) has been successfully applied to infer the various intramolecular vibrational [5], rotational [6, 7] or hindered rotational modes [8]. However, these methods lack the precision to quantify the interplay between structure and molecular geometry like methods such as electron spin resonance (ESR) [9, 10]. These methods are also not well suited for studying low-energy dynamics, such as the quantum zero-point motion of hydrogen and other light elements that are quenched by strong tip-sample interactions. Moreover, the resolution of traditional SPM, particularly scanning tunneling microscopy (STM), is limited by both convolution [1, 11, 12] and current preamplifier related bandwidth issues that preclude insight into the structure and rotational dynamics of individual molecules. Hybrid methods have recently emerged, combining the spatial resolution of STM with temporal resolution [13, 14] driven by continuous wave excitation [15]. THz-based STM [16, 17] has been used to excite and quantify the vibrational motion of an individual phthalocyanine molecule with picosecond precision [18]. Likewise, electron paramagnetic/spin resonance has been established [15, 19, 20], based on a combination of microwave excitation of the STM junction, with the detection of spin-polarized current [21] of individual atoms. This technique, referred to as ESR-STM, has been used to quantify magnetic interactions, hyperfine couplings, and the coherent dynamics of individual magnetic impurities with unprecedented resolution [22-24]. However, in the spirit of traditional EPR/ESR, ESR-STM has yet to be applied to infer the molecular str...

Combining Electron Spin Resonance Spectroscopy with Scanning Tunneling Microscopy at High Magnetic Fields

“…e baseline has been o set to zero for all sweeps. We nd a fairly even distribution of suitable frequencies across a magnetic eld range of almost 1.5 T. Such a spread allows for a more accurate determination of the 𝑔-factor, which contains valuable information on the molecule and its environment [32].…”

“…5). TiH adsorbed on the bridge site (between two oxygen atoms) of MgO is a spin-1 ⁄ 2 system with a 𝑔-factor close to 2 in an out-of-plane eld [6,[30][31][32][33]. We a ach one or more Fe atoms to the STM tip to generate a spin-polarised probe by picking them up from the surface [3].…”

“…e tip elds are 95.7 mT (B sweep) and 95.1 mT (𝜈 sweep) at 175 pA current setpoint and 46.1 mT for the 50 pA current setpoint. e small changes in the 𝑔-factor can be a ributed to tip-induced changes in the TiH bond length [32,33].…”

Double Electron Spin Resonance of Engineered Atomic Structures on a Surface

“…Individual spins, localized in atoms or molecules on surfaces, are a new candidate for realizing qubits, where atomically-precise fabrication can be achieved using a scanning tunneling microscope (STM). Quantum control of single atoms on surfaces driven by a magnetic tip has been demonstrated through continuous-wave (7)(8)(9)(10)(11) and pulsed (12) electron spin resonance (ESR) in STM. The STM can readily assemble multi-spin quantum systems (13,14) but use of ESR to coherently control a spin that is remote from the tunnel junction has not yet been considered (15)(16)(17)(18).…”

Harnessing the Quantum Behavior of Spins on Surfaces

“…These stringent requirements are met by drawing on powerful methodologies from material and quantum sciences, i.e., STM's abilities to build nanostructures atom-by-atom and selectively sense individual spin-carrying atoms, as well as ESR's ability to coherently control electron spin states via electromagnetic waves. This unique combination has so far enabled the quantum control of single electron spins of atoms [29,30,31,32,33] and molecules [34], as well as the manipulation of single nuclear spins through hyperfine interactions [35]. Quantum coherence can be increased using singlet-triplet states of a coupled spin system [36], allowing the observation of a free coherent evolution in the singlet-triplet basis [33].…”