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A 350 mK, 9 T scanning tunneling microscope for the study of superconducting thin films on insulating substrates and single crystals
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Image of FIG. 1.

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FIG. 1.

3D view of the LT-STM assembly consisting of three primary sub-units: (i) The sample preparation chamber, (ii) the load lock chamber to transfer the sample from the deposition chamber to the STM, and (iii) the 4He Dewar with 9 T magnet housing 3He cryostat on which the STM head is attached. The 4He Dewar hangs from a specially designed vibration isolation table mounted on pneumatic legs. The Dewar, cryostat and magnet have been made semi-transparent to show the internal construction.

Image of FIG. 2.

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FIG. 2.

3D view showing the construction of the STM head with the coarse positioner, piezoelectric scan-tube mounted, tip holder, and sample holder with the sample facing down. The main body of the STM head is made of gold-plated copper.

Image of FIG. 3.

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FIG. 3.

Design of sample holder (a) Molybdenum cap, (b) Substrate with strip deposited at the edge, (c) Molybdenum sample holder, (d) Sample holder assembly, showing substrate fastened with cap; (e) Resulting film on the substrate after the deposition.

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FIG. 4.

Schematic 3D view of the sample preparation chamber and load-lock cross. The deposition chamber incorporates two magnetron sputtering guns, a substrate heater for heating the substrate up to 800 °C, a plasma ion etching gun and two thermal evaporation sources. The substrate is inserted inside the deposition chamber using the horizontal manipulator.

Image of FIG. 5.

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FIG. 5.

Design of the horizontal sample manipulator with in-built thermocouple for measuring the temperature during sample deposition. A differential pumping arrangement between two Wilson seals is used to remove any leaked gas during movement. The end of the manipulator is made transparent to show the position of the thermocouple. The vertical manipulator is similar in construction but does not have the thermocouple.

Image of FIG. 6.

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FIG. 6.

Schematic view of the 3He cryostat inside the 4He dewar showing the sorption pump, 1 K pot, 3He pot and the STM head which is bolted below the 3He pot. The liquid 4He Dewar has a capacity of 65 l and a retention time of 5 days.

Image of FIG. 7.

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FIG. 7.

(a) Spectral density of the velocity vs. frequency on the top of the cryostat measured using an accelerometer. The spectral densities with and without the 1 K pot pump on are nearly identical. (b) Spectral density of the tunneling current with the tip out of tunneling range, within tunneling range with feedback on and with feedback off. (c) Spectral density of Z height signal with feedback on. Measurements in (b) and (c) were performed at 350 mK on a NbSe single crystal with tunneling current set to 50 pA and bias voltage to 20 mV.

Image of FIG. 8.

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FIG. 8.

Tunneling spectroscopy on Pb single crystal acquired with Pt-Ir tip at 500 mK along with BCS fit. The spectrum is averaged over 10 voltage sweeps at the same point. The spectroscopy set point before switching off the feedback was V = 6 mV, I = 500 pA, and the lock-in modulation voltage was 150 V with frequency of 419.3 Hz.

Image of FIG. 9.

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FIG. 9.

(a) Atomically resolved topographic image of NbSe obtained in constant current mode; the charge density wave modulation is also visible. The tunneling current was set to 150 pA, the bias voltage to 20 mV, and the scan speed was 13 nm/s. (b) Line cut along the line shown in (a).

Image of FIG. 10.

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FIG. 10.

Vortex imaging on NbSe. (a)-(d) 64 × 64 conductance maps over 352 × 352 nm area at different voltages at 350 mK in an applied field of 200 mT. The maps are obtained from full spectroscopic scans from −6 mV to 6 mV at each pixel. (e) Line scan of the tunneling spectra along the white line marked in panel (a). Three spectra inside (2) and outside (1 & 3) vortex cores are highlighted in black. (f) High resolution conductance map acquired over the same area by scanning at fixed bias of V = 1.4 mV; the tunneling current was set to 50 pA and modulation voltage was set to 150 V with frequency of 2.3 kHz.

Image of FIG. 11.

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FIG. 11.

STS data for NbN thin film with T = 6.4 K. (a) The topography of the surface over 200 × 200 nm area acquired in constant current mode. The tunneling current was set to 400 pA, the bias voltage to 8 mV, and the scan speed was 30 nm/s. (b) Normalized tunneling spectra along the horizontal dashed line in panel (a). (c) Normalized average tunneling spectrum over 200 × 200 nm area. The modulation voltage was set to 150 V and modulation frequency to 2 KHz.

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/content/aip/journal/rsi/84/12/10.1063/1.4849616
2013-12-26
2014-04-24

Abstract

We report the construction and performance of a low temperature, high field scanning tunneling microscope (STM) operating down to 350 mK and in magnetic fields up to 9 T, with thin film deposition and single crystal cleaving capabilities. The main focus lies on the simple design of STM head and a sample holder design that allows us to get spectroscopic data on superconducting thin films grown on insulating substrates. Other design details on sample transport, sample preparation chamber, and vibration isolation schemes are also described. We demonstrate the capability of our instrument through the atomic resolution imaging and spectroscopy on NbSe single crystal and spectroscopic maps obtained on homogeneously disordered NbN thin film.

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Scitation: A 350 mK, 9 T scanning tunneling microscope for the study of superconducting thin films on insulating substrates and single crystals
http://aip.metastore.ingenta.com/content/aip/journal/rsi/84/12/10.1063/1.4849616
10.1063/1.4849616
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