Detection of the Kondo Effect via Thermocurrent Spectroscopy
Jülich, 11 April 2022 - The Kondo effect is a ubiquitous phenomenon in condensed matter physics. It influences the electrical resistance of metals at low temperatures and could form a basis for new concepts of data storage and processing, for example, in the context of quantum dots. Now, an international research team including a theoretical physicist from Forschungszentrum Jülich has put forward a new method by which the Kondo effect in quantum dots can be detected and studied more easily and reliably than in previous approaches.
The Kondo effect occurs in certain types of quantum dots. These quantum dots typically consist of between a thousand and ten thousand atoms of one semiconductor embedded within another. Certain organic molecules can also function as quantum dots. The small size of these structures facilitates novel properties through quantum mechanical effects, which are the basis for their use in electronic and optoelectronic devices.
In semiconductor quantum dots with a finite magnetic moment, electrical conductivity increases with decreasing temperature due to the Kondo effect, in contrast to non-magnetic quantum dots. This unusual behaviour arises because a resonant state occurs at extremely low temperatures between the magnetic moment of the quantum dot and the magnetic moments of the conduction electrons, known as the Kondo resonance.
"An interesting property of this resonance is its high sensitivity to parameters such as temperature, magnetic field, bias voltage and gate voltage", reports physicist Dr Theodoulos Costi from the Peter Grünberg Institute in Jülich. "This allows one to tune the conductance of this type of quantum dot, which is potentially useful for applications."
In order to identify the Kondo effect in quantum dots, researchers typically try to demonstrate the theoretically predicted sensitivity of the Kondo resonance to a magnetic field, and in particular to its splitting at a certain field value. However, with the methods used so far, most commonly involving differential conductance measurements, detection is technically challenging. The team of researchers from Jülich, Delft, Karlsruhe, Basel, Guangzhou and Leuven has now shown that detection is easier and more reliable with the help of a novel spectroscopy technique, known as thermocurrent spectroscopy.
In this approach, the researchers apply a small thermal bias across the quantum dot and measure the resulting thermocurrent as a function of the electrical bias-voltage and the magnetic field. The derivative of the thermocurrent at zero electrical bias-voltage changes sign at exactly the magnetic field strength where the Kondo resonance splits in a magnetic field, thus demonstrating this characteristic feature of the Kondo effect in a more reliable way than has previously been possible. The theory for the nonlinear thermocurrent in a magnetic field, developed by Dr. Theodoulos Costi for this project, showed excellent agreement with the experiment.
Original publication:
Magnetic-Field Universality of the Kondo Effect Revealed by Thermocurrent Spectroscopy;
Chunwei Hsu, Theo A. Costi, David Vogel, Christina Wegeberg, Marcel Mayor, Herre S. J. van der Zant, Pascal Gehring;
Phys. Rev. Lett. 128, 147701 (8. April 2022), DOI: 10.1103/PhysRevLett.128.147701
Further information:
Peter Grünberg Institut – Theoretical Nanoelektronics (PGI-2/IAS-3)
Contact:
Dr. Theodoulos Costi
Forschungszentrum Jülich
Theoretical Nanoelectronics (PGI-2/IAS-3)
Tel: 02461 61-4246
E-Mail: t.costi@fz-juelich.de
Press contact:
Angela Wenzik
Science Journalist
Forschungszentrum Jülich
Tel. 02461 61-6048
E-Mail: a.wenzik@fz-juelich.de