Quantum Coherence of Spins on Insulators


Introduction

Spins on surfaces are the main unifying research topic of QNS where we are investigating their properties with STM, ensemble-averaging surface science ESR as well as through their interaction with point defects in diamonds (NV center). The majority of this work is carried out either on metal surfaces in the case of rare earth spins or on thin insulating films grown on metals for the majority of QNS’s work. Spins on thin insulating films can be studied with STM, which requires a tunnel current to flow between tip and sample. Hence the insulating films have to atomically thin typically between one and four layers thick. However, the key spin properties of energy relaxation time (T1) and quantum coherence time (T2) are strongly reduced through the presence of the metal electrons.

In this part of QNS’s research portfolio we are aiming to work with spin qubits on thick insulating films or on bulk insulators. Hence we need a new research tool to measure individual spins. The atomic force microscope (AFM) is a cousin of the STM and is also part of the larger family of scanning probe microscopes. The AFM measures the force between a tip and the sample. We mount a conducting tip on the end of a small cantilever in a geometry known as the q-plus sensor. This allows us to measure forces in the pico-Newton range. In order to measure the magnetic properties of spin-qubits, we rely on spin-dependent forces between a magnetic tip and the qubits on the insulating surface.

Longer-term Goals

• Demonstrate coherent control of spin qubits on insulatorssurfaces
• Demonstrate long quantum coherence times for spin qubits on insulators

Near-term Goals

• Demonstrate magnetic exchange force microscopy
• Demonstrate simultaneous STM and AFM in magnetic fields
• Use AFM to image the structure of molecular spin qubits

Research results to date

Research results to date
Figure.1

Over the past three years or so, QNS has slowly advanced our state of the art in AFM by measuring molecules on surfaces as well as spins on thin insulating films. We plan to start measuring spins on thick insulating films in 2022.
In January 2021, QNS finished the construction of our low-temperature AFM, nicknamed “Oracle” that operates at low temperature and in high magnetic field. Oracle has a quick rotate-and-lock sample transfer mechanism and we can quickly pull up the sample while it is still cold. Thus we can deposit single atoms on the surface in our room temperature chamber. Oracle is fully equipped with STM and AFM operations at the same time. Figure 1 shows simultaneously obtained STM and AFM image of a single Fe atom sitting on MgO surface, which demonstrates the similar quality in atomic scale imaging. We are in the process to measure spin-dependent forces using spin-polarized tips in magnetic fields.

Figure.2
In 2019, QNS used AFM to characterize the interaction between hydrogen molecules and vanadyl-phtalocyanine (VOPc) molecules adsorbed on gold. We investigated the dynamic behavior of hydrogen molecules, which can be trapped and released between the tip and a VOPc molecule in the tunnel junction depending on the applied bias voltages. The trapped hydrogen molecule can be used to image the intra-molecular structure, which cannot be achieved by normal STM and AFM imaging. Figure 2 shows the AFM images of VOPc molecules using bias voltages of 60 mV (without hydrogen) and 10 mV (with trapped hydrogen), which clearly visualize the internal molecular structure. This result suggests that our AFM setup should have sufficient force sensitivity to detect spin-dependent forces from a single magnetic atom. The work was published in Royal Society of Chemistry Advances (2021).

*Fig.1 Simultaneously obtained STM and AFM image of a single Fe atom on MgO. Image size: (2nm)2, image obtained with our “Oracle” STM/AFM system.

*Fig.2 AFM images of a single VOPc molecule with bias voltage of 60 mV (left) and 10 mV (right) configurations on gold. Image size: (3nm)2, image obtained with our “xxxx” STM/AFM system.

Research Equipment

Adam LT-AFM system
FULLY home-built system
Delivery: April 2018
First Results: October 2018
Key results
• Iron atoms on thin NaCl insulating films
• Hydrogen interaction with VO-Pc molecules
Specs:
Temp: 8K when flowing liquid helium 85K when flowing liquid nitrogen
AFM: Q-plus force sensor
4 T in plane
Home-built cryogenic AFM preamplifier
Figure.3 Photo of AFM/STM module and preamplifier.

Adam is our first AFM/STM system, which was refurbished and upgraded from an old STM from Prof. Young Kuk’s lab in Seoul National University. Adam has a continuous flow cooling system where AFM/STM is thermally connected to a small cryostat through gold plated copper rods and wires. The baseplate of the AFM/STM is hanging by three springs with an eddy current damper at the lower part of the module to block mechanical vibrations. Two gold plated copper boxes enclose the AFM/STM to reduce thermal radiation. We achieve 8 K at the AFM/STM when cooling with liquid helium (85 K with liquid nitrogen).

Adam uses a q-plus sensor with attached metal tip to operate AFM and STM simultaneously. The q plus sensor generates charge signals when it is deflected which are converted to voltages in our electronics. QNS built our own preamplifier, which is designed to operate at low temperature to reduce electric noise. The preamplifier is installed inside the wall of the outer shield, where the temperature is about 100K. Figure 3 shows the AFM/STM inside the shields with the preamplifier on top.

Adam was installed in April 2018 in our temporary lab space and moved to our new building in the summer of 2019. Adam was a proto-type system for AFM functionality in QNS and it demonstrated the capability of force sensing. Adam is a simple-to-use system compared to other STMs in QNS and it is dedicated to optimizing the preparation of new samples such as graphene grown on platinum.


*Fig.3 Photo of AFM/STM module and preamplifier. AFM module is hanging by three springs inside two gold plated copper boxes. The top part shows the home-built cryogenic AFM preamplifier installed on the inside wall of the outer shield.
Oracle LT-AFM system
FULLY home-built system
Delivery: September 2019
First Results: February 2021
Key results
• Iron atoms on thin MgO insulating films
Specs:
Temp: 8K
Magnet: 8T perpendicular to sample surface
AFM: Q-plus force sensor
Home-built cryogenic AFM preamplifier

Oracle is our second AFM/STM system, which was also refurbished and upgraded from an old STM from Prof. Young Kuk’s lab at Seoul National University. Oracle consists of two vacuum chambers separated by a gate valve. The preparation chamber is used to clean substrates and grow insulator films. The main chamber has two parts. One is the room temperature part at the top, where single atoms are evaporated, and the other is the low temperature part for AFM/STM, see figure 4. Oracle has a dewar-type cooling system, where the AFM/STM is sitting 1m deep inside the dewar that is filled with liquid helium and contains a single axis superconducting magnet. Once the dewar is filled, it holds 3 days at 8K. Oracle uses helium exchange gas, which is inserted between the liquid helium bath and the AFM/STM chamber in order to enable thermal conduction while isolating the bubbling noise from the liquid cryogens. The same cryogenic preamplifier as Adam’s is installed 30cm above the AFM/STM inside the exchange gas can for AFM.

Figure.4 Photo of Oracle.

Oracle was moved to QNS in September 2019 but we waited for the AFM demonstration using Adam. We started to assemble it in July 2020 and finished the STM test in February 2021 including spin-polarized tip fabrication. We are measuring spin-dependent forces on single magnetic atoms on a thin MgO film. Once we can demonstrate the spin sensitivity, we are planning to equip Oracle with ESR capabilities. If we can succeed in the near future, it will be the first ESR-AFM system in the world. We are looking forward to make another breakthrough in QNS!




*Fig.4 Photo of Oracle. Left part is to load a new sample and the middle part is a preparation chamber to clean samples and grow insulator films. The long tube on the right part is the sample transfer rod to move a sample into AFM/STM sitting 1m deep inside a dewar. The right part is the room temperature region where we can evaporate single atoms. The dewar is connected below the aluminum plate at the bottom.
X