Scanning Tunneling Microscopy in high vectorial magnetic fields

Abstract

The Scanning Tunneling Microscope (STM) is a powerful instrument to study electronic density of states at surfaces down to atomic scale. Many interesting samples require studying variations as a function of the magnetic field, which is most often applied perpendicular to the surface. Conventional STM designs make it challenging to perform measurements when the magnetic field must be applied in other directions. Here we present a new STM setup installed on a rotatable platform. We have designed and built a new STM, which is small enough to allow for full rotation on a space with a diameter of 37 mm, well below the available space within many magnets. We show that the new rotatable STM setup preserves the performance of state-of-the-art STMs in terms of noise and accuracy. Our new approach significantly enhances control over the direction of the applied magnetic field and opens exciting new possibilities to study quantum materials.

Figures (7)

• Figure 1: We schematically show a rotatable Scanning Tunneling Microscope (STM) as a black rectangle. Within the microscope we represent schematically a scanning piezoelement (grey), a tip (orange) and a surface (orange spheres schematically representing atoms). The solenoid is shown in red. The direction of the applied magnetic field is the red arrow. The direction of the STM is varied using a rotatable platform, with the main $x$ and $z$ axis within the surface frame shown in green. The rotating angle is $\theta$.
• Figure 2: (a) Schematic of the setup, positioned such that the magnetic field is perpendicular to the surface. A is the rotatable platform. B are copper beams firmly attached to the platform. C are teflon disks which allow to firmly attach the rotatable platform to the copper beams, still allowing for in-situ rotation at cryogenic temperatures. (b) Same schematic as (a) with the platform rotated such that the magnetic field is tilted by 60° from the surface. D is a steel wire. E is a kevlar rope. F is a counteracting spring. (c) Schematic of the vacuum feedthrough actuator. G is a screwed bar. H is a bellow. I is a gearwheel. (d) Schematic drawing of the miniaturized STM. J is the STM base. K is the walker. L is the piezotube. M is a CuBe spring used to fix the walker to the shear piezostack. N is the main head of the miniaturized STM. O are the piezo-stacks for the coarse motion. P are polished alumina. (e) Profile section of the STM base (J) and the sample showing the cleaving process. Top picture shows the sample prepared before being cleaved. Q is a steel wire attached to a second actuator. R is a beam used to cleave the sample. S is the alumina glued on the sample surface. T is the slider. Bottom picture shows the sample at the moment of being cleaved. U is the sample cleaved. V is the cleaved part of the sample.
• Figure 3: (a) We show with a color scale the deformations of the miniaturized STM at its first resonance frequency, 11.4 kHz, obtained using finite element analysis with Siemens NX. The STM setup is drawn distorted, with an exaggerated displacement to highlight the motion corresponding to this first resonance mode. (b) Transfer function $\frac{V_{out}}{V_{in}}(\omega)$ as a function of frequency $\omega$ of the miniaturized STM obtained as described in the text. In the top left inset we show the portion of the curve inside curve the black-square of the main panel. We show with arrows and numbers the first three features observed in the transfer function. In the bottom right inset we show the derivative of the transfer function within the same frequency interval. We again show the first three resonance frequencies with arrows and numbers. These are located at 13.7 kHz, 18.3 kHz and 27.0 kHz. (c) Same as (b), but for a STM with a diameter about twice larger (see text for more details). In that case, due to the larger size of the STM, the first resonances are located at lower frequencies, 9.0 kHz, 12.1 kHz and 13.8 kHz.
• Figure 4: Vibration velocity as a function of frequency with the platform rotated 0$^{\circ}$, 45$^{\circ}$ and 90$^{\circ}$. The velocity is obtained from the Fourier transform of the voltage signal measured with the accelerometer, as described in the text.
• Figure 5: (a) Lateral view of the whole setup. The cryostat (A) is inserted in a hole in the ground (grey) and suspended on pneumatic dampers for vibration isolation (B). A 9 T superconducting magnet is located at the bottom of the cryostat (C). (b) Schematic drawing of the insert. D and E show two equivalent vacuum feedthrough actuators described more in detail in Fig. \ref{['Plataforma']} (c). F, G and H are the wire feedthroughs. I are stainless steel tubes containing the wiring. J are radiation shields. K are copper support beams holding the rotatable platform. L is the miniaturized STM located within the rotatable platform. M is the tube with an indium seal making the inner vacuum chamber.