NbSe2

Niobiumdiselenide is a type II superconductor (SC) which also exhibits a charge density wave (CDW). The transition temperatures are TSC=7.2 K and TCDW=33 K, respectively.

Atomic structure of NbSe2


Topography

The topography shows large monatomically flat terraces separated by steps of ~0.6 nm in height. On the terraces a triangular lattice of the selenium atoms can be observed. Every third atom is brighter due to the charge density wave.

1 nA, 20 mV 0.2 nA, -50 mV


Spectroscopy

Spectroscopic measurements reveal the energy gap of the charge density wave as well as the superconductive bandgap.


Modulation 1 mV



Modulation 0.038 mV, temperature 2 K


Vortices

When a magnetic field above a threshold is applied to a type II superconductor, part of the field enters the SC in form of threads in a region of normal conductance surrounded by a superconductive shielding current. This structure is called a vortex. Several methods have been used to image vortices which have all their own advantages and disadvantages. Optical methods are limited in resolution while scanning probe methods usually have a limited field of view and temporal resolution.

To image vortices using STM we used the difference of the density of electron states between the SC and normal phase. In the magnetic field range we used, the vortices form a triangular lattice due to the strong repulsive vortex-vortex interaction.

Difference in spectroscopy. Spectroscopic Image (400x400 nm)


Vortex Motion

We found, that the vortices move slowly across our field of view. In order to study this phenomenon in detail we took series of up to 2560 images. The Images were automatically processed to extract the vortex position and to track individual vortices throughout the data set.

The Cause

The magnetic field of our superconductive magnet slowly decays due to residual resistances presumably at spot welds within the solenoid. As a result, vortices have to leave the sample, which happens at its edge. With the scan area somewhere between the center of the sample and the edge vortices should move in a certain direction at a near constant velocity.

Animated vortex model (6.5 MB)

Automated Vortex Tracking

Original image. Typically 128x128 pixel,
400x400 nm2 => 3.125 nm2/Pixel
Preprocessing:
  • Background subtraction
  • Filtering
  • Inverting
Single threshold to find vortex area.
Find center of vortices as weighted sum
=> resolution increases to ~ 0.36 nm.
Extract center position and time when
the tip moved across that point.
Calculate vortex lattice within one frame.
Use consecutive frames to extract
vortex tracks and velocities.


Examples of vortex motion

Here are three examples of data series gathered on different samples and using different STM tips.

Vortex movie 1 (5.2 MB) Vortex movie 2 (9.5 MB) Vortex movie 3 (8.4 MB)

Local lattice distortion

The moving vortex lattice can be used to probe the sample it is moving through. The high lateral resolution allows us to locate single point defects by looking for local distortions of the vortex lattice.

Combined tracks of three consecutive data series. The arrows mark the lattice distortion.

Local distortion marking the position of a point defect.