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...

In the last decade, detecting spin dynamics at the atomic scale has been enabled by combining techniques such as electron spin resonance (ESR) or pump-probe spectroscopy with scanning tunneling microscopy (STM). Here, we demonstrate an ultra-high vacuum STM operational at milliKelvin (mK) temperatures and in a vector magnetic field capable of both ESR and pump-probe spectroscopy. By implementing GHz compatible cabling, we achieve appreciable RF amplitudes at the junction while maintaining the mK base temperature and high energy resolution. We demonstrate the successful operation of our setup by utilizing two experimental ESR modes (frequency sweep and magnetic field sweep) on an individual TiH molecule on MgO/Ag(100) and extract the effective g-factor. We trace the ESR transitions down to MHz into an unprecedented low frequency band enabled by the mK base temperature. We also implement an all-electrical pump-probe scheme based on waveform sequencing suited for studying dynamics down to the nanoseconds range. We benchmark our system by detecting the spin relaxation time T1 of individual Fe atoms on MgO/Ag(100) and note a field strength and orientation dependent relaxation time

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BCS theory has been widely successful at describing elemental bulk superconductors. Yet, as the length scales of such superconductors approach the atomic limit, dimensionality as well as the environment of the superconductor can lead to drastically different and unpredictable superconducting behavior. Here, we report a threefold enhancement of the superconducting critical temperature and gap size in ultrathin epitaxial Al films on Si(111), when approaching the 2D limit, based on high-resolution scanning tunneling microscopy/spectroscopy (STM/STS) measurements. Using spatially resolved spectroscopy, we characterize the vortex structure in the presence of a strong Zeeman field and find evidence of a paramagnetic Meissner effect originating from odd-frequency pairing contributions. These results illustrate two notable influences of reduced dimensionality on a BCS superconductor and present a platform to study BCS superconductivity in large magnetic fields.