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

Magnetic media remain a key in information storage and processing. The continuous increase of storage densities and the desire for quantum memories and computers pushes the limits of magnetic characterisation techniques. Ultimately, a tool which is capable of coherently manipulating and detecting individual quantum spins is needed. The scanning tunnelling microscope (STM) is the only technique which unites the prerequisites of high spatial and energy resolution, low temperature and high magnetic fields to achieve this goal. Limitations in the available frequency range for electron spin resonance STM (ESR-STM) mean that many instruments operate in the thermal noise regime. We resolve challenges in signal delivery to extend the operational frequency range of ESR-STM by more than a factor of two and up to 100GHz, making the Zeeman energy the dominant energy scale at achievable cryogenic temperatures of a few hundred millikelvin. We present a general method for augmenting existing instruments into ESR-STMs to investigate spin dynamics in the high-field limit. We demonstrate the performance of the instrument by analysing inelastic tunnelling in a junction driven by a microwave signal and provide proof of principle measurements for ESR-STM.


INTRODUCTION
e direct manipulation and detection of individual spins (see Fig. 1(a)) is one of the major goals in contemporary nanoscience [1][2][3][4][5][6][7][8][9]. Meeting these challenges requires a local measurement of electronic and magnetic properties with atomic precision. e scanning tunnelling microscope (STM) routinely achieves this limit of resolution and is thus an ideal tool to study the dynamics of magnetic nano-objects [10] on their own length and time scales [11][12][13][14].
Combining electron spin resonance with STM (ESR-STM) has introduced new possibilities to the local studies of individual spins and has expanded the available parameters space substantially, but it imposes a series of strict experimental requirements, most notably on the base temperature of the cryostat. e operational frequency range of the instrument determines the maximum magnetic eld for ESR-STM experiments and sets the relevant energy scale in the experiment. ESR-STM relies on the thermal initialisation of the target systems into their ground state. However, in many contemporary implementations of ESR-STM, the Zeeman energy is on the order of and a non-negligible excited state population remains [4,6].
is is a signi cant impediment to resolving intrinsic spin dynamics at the nanoscale. e goal of coherent manipulation from a known ground state may be reached via two approaches: Reducing the base temperature of the experiment to suppress thermal excitations from the ground state, or increase the microwave frequency to operate at higher magnetic elds.
Current implementations of ESR-STM typically operate at frequencies up to 40 GHz [15][16][17]. To achieve thermal initialisation of the target systems at these frequencies mK temperatures are required, which are only achievable in dilution refrigerators [18]. is approach requires dedicated machines that are costly to produce and present signi cant challenges in everyday operation. High frequency signals in the upper GHz range, on the other hand, can be generated in an independent setup outside the ultra-high vacuum (UHV) system and routed to the tunnel junction through a set of suitable cables.
is approach is, therefore, more exible, allowing the retro ing or modi cation of existing machines by the addition of dedicated high GHz cabling [19].
Extending the operational frequency range of ESR-STM has thus far been prevented by challenges in signal delivery. We have augmented a commercially available STM (Unisoku model USM1300) featuring 310 mK base temperature and a 6 T single axis magnet with an antenna assembly which permits us to deliver microwave signals of up to 105 GHz directly to the tunnel junction. is work can be used as a guideline to design new instruments or retro t existing ones for high GHz microwave capabilities.
INSTRUMENT DESIGN e Unisoku model USM1300 STM is a commercially available experimental platform combining ultrahigh vacuum (UHV) sample preparation and ultra-low temperature STM with high eld capabilities. e STM unit, developed and manufactured by the Unisoku corporation, is installed on the insert of a superinsulated 4 He bath cryostat produced by Cryogenic Ltd. e insert includes a 1 K pot, which is supplied with liquid helium from the bath through an adjustable needle valve, and a single-shot 3 He cooling cycle. With a total volume of 30 l 3 He gas, the STM is capable of operating at a base temperature of 310 mK for up to 72 hours. Figure 1(b) shows a sketch of the cryostat insert including the modi cations we implemented as part of the ESR-arXiv:2111.05910v1 [cond-mat.mes-hall] 10 Nov 2021 e main addition to the base setup is the installation of a series of 0.047 inch semirigid coaxial cables, rated to 110 GHz, and a radio frequency antenna into the system. We solve the challenges of nding leak-tight vacuum feedthroughs and thermalisation of the RF assembly to produce a high-performance machine capable of delivering high-frequency signals at large amplitudes onto the tunnel junction. Our approach extends the operational frequency range of ESR-STM by a factor of more than two while maintaining signal amplitudes comparable with previous e orts. Figure 2. Transmission through the 1.85 mm feedthrough KPC185FFHA, rated to 65 GHz, (blue) and the 1 mm female-female adapter 33WFWF50, rated to 110 GHz. Bot components have comparable losses in the frequency range of interest.
Below is a step-by-step discussion of the design philosophy and implementation of our custom modi cations to the base system.
High-frequency wiring e geometry of STMs presents a severe challenge to the integration of high-frequency signals. e con ned space of the scanner housing and complex shape of the tip-sample system thwart any a empt to realise an impedance-matched connection to the tip. e resulting sca ering of electromagnetic waves in the instrument will inevitably lead to high losses. is makes it all the more di cult to bring a high amplitude signal close to the tunnel contact. e requirement for high signal amplitudes leads to a con ict with a key design principle for cryostats to use high-resistance cabling in order to limit the thermal load on the experiment. We overcome these issues by using a combination of high conductance coaxial cables from di erent materials to achieve maximum power transmission to the antenna. We use semirigid cables with a copper jacket and silver-plated copper weld (SPCW) conductor on the air side and in the upper sections of the cryostat to the 1K-pot (see red wire section in Fig. 1(b)). eir high conductance ensures small signal losses even at room temperature. We installed a coaxial cable with NbTi shield and conductor running from the 1K-pot to the 3 He stage (see blue wire section in Fig. 1(b)). NbTi is a superconductor with a transition temperature of 10 K and a high critical eld of 15 T. As superconductors are excellent conductors of electricity, but very poor conductors of heat [20], the NbTi cable provides excellent signal transmission at low temperatures while essentially eliminating thermal loads on the low-temperature parts of the experiment. Finally, a exible coaxial cable with silver plated copper shield and conductor connects the RF antenna in order to preserve STM motion during sample transfer and spring damping during regular operation.
We use semirigid coaxial cables with 0.047 in outer diam-eter for all applications. is cabling standard is rated to 110 GHz with an impedance of 50 Ω. We installed 1 mm connectors (Anritsu W1 series), also rated to 110 GHz, on all cable segments. Before installation in the machine, all cables were repeatedly immersed in liquid helium and rigorously tested for any temperature related damages.
Our combination of copper/SPCW and superconducting cables requires excellent heat management in order to work e ectively. To ensure proper thermalisation, all cables are anchored at several points inside the cryotat. e thermal anchors consist of a copper braid, glued to the outer conductor of the cable over a large surface area using thermally conductive silver epoxy, and fastened to the anchor points with a screw and lug. e copper/SPCW cable is anchored at the ba es of the cryostat, the sorption pump, and the 1Kpot. e NbTi cable is thermally anchored at the 1K-pot and 3 He-pot as described above and wound in a wide loop around the UHV column to accommodate the thermal expansion and contraction of the wiring during cool-down or warm-up. e installation of our custom radio frequency cabling did not affect the base temperature of the instrument.

High-frequency UHV feedthrough
To our knowledge, there are no commercially available hermetically sealed double ended coaxial vacuum feedthroughs rated to 90 GHz or above. In practice, however, vacuum feedthroughs with 1.85 mm connectors, rated to 65 GHz, show very low losses up to at least 90 GHz and can act as a substitute. Fig. 2 shows transmission measurements through the hermetically sealed feedthrough KPC185FFHA by Kawashima Manufacturing Corporation from 60 GHz to 90 GHz. e performance of KPC185FFHA is comparable to 1 mm female-femals adapters rated to 110 GHz. A pair of 1.85 mm to 1 mm adapters (e.g. CentricRF C8186) is needed to mate the feedthrough to the high-frequency cabling.
High-frequency antenna e implementation of the dedicated high-frequency line requires a solution for coupling the high-frequency signal to the tunnel junction. We designed an antenna to transform the electric signal in the high-frequency line into electromagnetic radiation illuminating the tunnel junction, with the STM tip e ectively acting as a receiver. A dedicated antenna ensures e cient coupling into the vacuum, eliminating losses. Mounting it in close proximity to the tunnel contact further improves the signal strength.
We chose an on-chip antenna design for a compact and integrated solution [21]. e silicon chip is mounted in a phosphorous bronze carrier a ached directly to the side of the STM scanner housing, shown in Fig. 3(a). A ange mount connector (Anritsu W1-103F) provides the electrical contact to the high-frequency wiring. To increase the microwave power incident on the tunnel junction, we ed the carrier with a hyper-hemispherical silicon lens ush with the underside of the antenna chip to partially collimate the radiation.
A broadband bowtie antenna a ords the most exibility, covering the entire intended frequency range of 60 to 90 GHz. Fig. 3(b) shows a dimensional drawing of the optimised antenna structure installed in the microscope. e antenna is assembled in a thin AuPd lm on a high resistivity silicon substrate of 380 μm thickness. Design and parameter optimisation for the antenna was performed using the CST Microwave Studio so ware with performance tests in 8.4:1 scale models. Fig. 3(c) shows the simulated re ectance of the antenna from DC up to 120 GHz [21]. e design achieves excellent power dissipation across a wide band beginning at about 60 GHz.

RADIO FREQUENCY GENERATION
We use a multi-stage generation scheme to reach our intended operational frequency window (see Fig. 1(b)). e rst stage is a baseband generator (Keysight 8257D) capable of producing signals up to 20 GHz.
is generator feeds a frequency extension module (VDI WR12SGX), which multiplies the input frequency by a factor of six. e frequency extension module is intended to operate with input frequencies between 10 and 15 GHz, producing a constant amplitude signal between 60 and 90 GHz. We found that upper limit can be extended by feeding the module with higher input frequencies, but at a signi cant cost in amplitude. Still, overdriving allows us to extend the operational frequency range of our ESR-STM to 105 GHz. Expanding the frequency range at the lower end in the same fashion is not possible due to the sharp cut-o of the WR12 waveguide output on the extension module. We regulate the source amplitude through a computer-controlled rotary vane a enuator (Mi-Wave 511E/387ND). is device allows us to regulate the power entering the high-frequency wiring in steps of 0.1 dB from the source power of the frequency extension module.

INSTRUMENT PERFORMANCE
Key operational parameters of the instrument, such as base temperature, -stability and 3 He hold time, were una ected by our modi cations. We demonstrate the excellent STM performance in a series of test measurements on V(100). Fig. 4(a) shows the oxygen reconstruction of the V(100) surface [22,23] in atomic resolution at the base temperature of 310 mK a er the system upgrade. Tunnelling spectroscopy between a superconducting vanadium tip and the V(100) sample shows well-developed coherence peaks and a clear energy gap typical of superconductor-insulatorsuperconductor junctions (see Fig. 4(b)) [20,24,25].

Microwave signal delivery
We evaluate the performance of the microwave assembly by observing microwave-assisted tunnelling processes occurring in a superconductor-insulator-superconductor (SIS) junction between a V(100) sample and a vanadium tip driven by a high-frequency signal from the antenna. SIS junctions are well studied and a robust framework for data analysis is readily available [19,[26][27][28]. e current in presence of a radio frequency signal is [26,29]: where QP ( , 0) is the quasiparticle current in absence of any RF radiation, 0 the DC bias voltage applied to the tunnel junction, the AC voltage dropping across the junction as a result of the RF signal, are the Bessel functions of the rst kind of order , and is the elementary charge. Further, ℏ is the reduced Planck constant and = 2 , where is the microwave frequency. e sharp coherence peaks in the SIS tunnel data allow us to observe directly the e ects of changing frequency. To analyse our data, we use reference spectra acquired using the same parameters as the corresponding microwave-assisted tunnelling spectrum, but with the RF signal switched o . is reference measurement is used as input for in Eq. (1). Fi ing our model to the experimental data allows us to extract the frequency and amplitude of the RF signal arriving at the junction. All data shown is acquired at a base temperature of 310 mK.  We t our data with the model given in Eq. (1) to extract the AC voltage dropping across the RF driven junction. Fig. 4(c) shows sample ts for a junction under irradiation by 60 GHz radio waves. As the RF power, and hence the AC voltage drop across the SIS junction, increase, higher order processes become visible. As the total current through the junction remains the same, microwave-assisted tunnelling leads to an overall decrease of the heights of individual peaks as the spectral weight is redistributed across a wide voltage range. e ed curves are in excellent agreement with the data. Only a single frequency is needed in the model to reproduce the experimental results [19].

ESR-STM
e prototypical spin systems for ESR-STM are individual metal atoms on an MgO decoupling layer on Ag(100) [3]. We grow double-layer MgO islands by sublimation of Mg in a 1×10 −6 mbar oxygen atmosphere onto the sample held at 700 K. Sublimation of Ti and Fe from an electron beam evaporator onto the cold sample (T 20 K) yields individual Fe atoms and TiH molecules. We use individual metal atoms on MgO/Ag(100) as a model system to provide proof of principle ESR-STM measurements. Fig. 5(a) shows an STM topograph of a typical sample, once again demonstrating that the machine can resolve single atoms with ease. e tip is a sharpened PtIr wire, further prepared by eld emission in the microscope and repeated indentation into the Ag(100). We identify individual Fe atoms and TiH molecules through their ngerprints in inelastic excitation tunnel spectroscopy (IETS), see Fig. 5(b) and (c). e essential ingredient of any ESR measurement is the tuning of the spin system across its resonance with the microwave signal. is may be achieved through either changing the excitation frequency at xed eld ( sweep), or the external eld at xed excitation frequency ( sweep). In either case, it is necessary to calibrate the microwave amplitude in order to perform consistent and comparable measurements. Frequency sweeps further require the compensation of the antenna-junction transfer function to ensure a constant amplitude signal at throughout the measurement to suppress spurious signals.
We mainly follow Ref. [15] to calibrate the microwave signal generate constant amplitude radio frequency sweeps at the junction. A similar procedure is given in Ref. [6]. Indi- vidual TiH molecules on MgO show a prominent step-like feature at a bias voltage of about −80 mV (see blue spectrum in Fig. 6(a)). We rst determine the transfer function of the antenna assembly at one xed frequency using Eq. (1). We acquire a reference spectrum in absence of any RF signal and a tunnel spectrum under RF irradiation, as for the SIS case above. en, we extract the signal amplitude through a t (see yellow spectrum in Fig. 6(a)) of the data for the irradiated junction (see red spectrum in Fig. 6(a)). e transfer function is then easily found from the known source amplitude S according to = 20 log( / S ). ( We then acquire a calibration curve by measuring the conductance near the centre of the slope of the TiH step as a function of source amplitude, see Fig. 6(b). We use a smoothing cubic spline interpolation to t the calibration function more accurately for arbitrary shapes. e smoothing parameter must be calculated such that the resulting smoothed spline can be inverted.
is produces be er ts than directly t-ting to the inverted data. With the known transfer function value, these values can be converted into signal amplitudes at the junction. Since the TiH step is much broader than ℎ in the frequency range of interest, the frequency itself has no measurable e ect on the RF spectrum and the calibration curve becomes a universal injective mapping of the conductance on . By measuring the conductance of our tunnel junction with xed signal a enuation while varying the source frequency and mapping the acquired values onto amplitudes using the inverted calibration curve, we measure the transfer function of the antenna assembly over a large frequency range, see Fig. 6(c). e instrument performs well over the complete intended operational range of 60 to 90 GHz and beyond. Losses do not become insurmountable until about 105 GHz. Using the experimentally determined transfer function, we can compensate the losses in the antenna assembly through the adjustable a enuator to generate constant amplitude radio frequency sweeps at the tunnel junction, see Fig. 6(d). We achieve signal amplitudes of 10 mV throughout most of the frequency range, and up to about 90 mV at select frequencies.

ESR Signal
We demonstrate the ESR capabilities of our setup using TiH molecules on double layer MgO/Ag(100) as a model system (see Fig. 5). TiH adsorbed on the bridge site (between two oxygen atoms) of MgO is a spin-1 ⁄ 2 system with a -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]. An out-of-plane magnetic eld li s the degeneracy between the spin states of the TiH molecule. In our frequency range, resonance is achieved in elds between about 2.15 T and 3.75 T. e Zeeman energy in these elds is about an order of magnitude larger than the thermal energy at the base temperature of 310 mK and the spin systems are thermally initialised to their ground state. All of the data presented here are measured at a bias voltage of 100 mV and the setpoint currents are referenced to this voltage. e RF signal drives the resonant transition between the ground and excited states, resulting in a reduction of the spinpolarised signal on the atom as the time-averaged spin population is no longer thermal. By chopping the RF driving signal and locking in to the chopping frequency with a lock-in ampli er, we single out and record those parts of the signal that are a ected by the driving, i.e. the ESR-STM signal [3,15].
Our results are summarised in Fig. 7. All data shown was acquired with the same spin-polarized microtip on the same TiH molecule adsorbed on a bridge site. Figure 7(a) and (b) show frequency sweep ESR-STM measurements at 175 pA and 50 pA setpoint current, respectively. e RF amplitudes are 10 mV and 15 mV for the data at 175 pA and 50 pA, respectively. We nd reasonably spaced intervals throughout the frequency band between 60 GHz and 95 GHz, where a sweep is possible (as indicated in the insets). As the probe current is itself a source of decoherence, the line width is substantially reduced at lower current.
is reproduces the general trend that has been observed previously [4]. Also, the line widths of the measured ESR peaks are comparable to the published literature at lower frequencies (≤ 40 GHz). Fig. 7(c) shows ESR data from sweep measurements at sixteen di erent excitation frequencies spanning the available frequency range between 60 GHz and 100 GHz of the instrument.
e microwave amplitude was set to 10 mV. 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 -factor, which contains valuable information on the molecule and its environment [32].
We calculate the corresponding Zeeman energies Z = ℎ res for all data sets, where res is the resonance frequency. e -factor of our spin system can then be extracted through a simple linear t according to where B is the Bohr magneton and = 1/2 is the spin of the TiH molecule. e results are shown in Fig. 7(d). We consistently obtain -factors close to 2 for all our measurements, in agreement with previous results for TiH on the bridge site and in an out-of-plane eld [6]. e data points lie very well on the line given by Eq. (3) for both the sweep as well as the sweep, which were extracted from independent measurements at 175 pA setpoint current shown in Fig.  7(a) and (c). e ed -factors are = 2.005 ( sweep) and = 2.008 ( sweep), which lie within 0.15%. For the sweep at 50 pA setpoint current, we nd a -factor of = 1.988, which is slightly smaller than for the higher setpoint current.
e o set between the sweeps at 50 pA and 175 pA setpoint current can be a ributed to the di erent tip elds tip felt by the TiH molecule. e tip elds are 95.7 mT (B sweep) and 95.1 mT ( sweep) at 175 pA current setpoint and 46.1 mT for the 50 pA current setpoint. e small changes in the -factor can be a ributed to tip-induced changes in the TiH bond length [32,33]. CONCLUSION We have augmented a commercially available lowtemperature STM system into a high-performance ESR-STM by the addition of a dedicated high-frequency line with antenna assembly to deliver radio frequency signals between 60 and 105 GHz to the tunnel junction. Using commercially available components rated to high GHz frequencies, we achieve a high signal amplitude across a wide frequency range. e compensation of the transfer function allows us to keep the signal amplitude constant throughout our frequency range. In a series of proof-of-principle measurements, we measure an ESR signal on individual TiH molecules on a MgO decoupling layer in both frequency sweep and eld sweep modes. In an operational eld of several Tesla and at a base temperature of 310 mK, excited state populations of typical spin-1 ⁄ 2 systems at a resonance frequency above 60 GHz are lower than 0.01%. With these parameters, it becomes possible to study the intrinsic dynamics of individual spins with atomic resolution. Our approach can serve as a template to convert typical STMs into ESR-STMs capable of resolving the coherent dynamics of individual spins and magnetic nanostructures.