Optical properties of SmB6 revisited

Despite a large global search, combining strong correlations and electronic topological twists in one material has so far been limited to the discovery of a single material: the Kondo insulator SmB6. We present new insights on the formation of the Kondo insulating state in our latest publication.

Topological twists in electronic band structures become noticeable at the boundaries of materials. At interfaces or surfaces, the electronic wavefunctions need to smoothly connect from one side to another. The new edge states that emerge are protected by the topological invariants on both sides of the boundary. SmB6 has been widely studied as an example in which heavy f-electrons and d-electrons interact, giving rise to the formation of a low temperature Kondo insulating state. The formation of the Kondo state coincides with a change in total parity of the occupied bands, resulting in an extra topological twist in the bandstructure. This is expected to give rise to topological edge states and possibly provides an explanation for a saturation of the resistivity at low temperature.

We show using optical spectroscopy that the formation of the Kondo state goes hand in hand with subtle shifts of optical spectral weight over extremely large energy ranges (of order U≈ 7 eV). Our results give an experimental estimate of the effective Coulomb interaction, but our detailed analysis also provides evidence that the low energy properties (such as the size of the energy gap) are set by the same energy scale.


First paper in Sci. Rep.

The first paper of the group has been accepted for publication in Scientific Reports. In this paper we show that the optical conductivity of the electron doped iron-pnictide superconductor BaFe1.8Co0.2As2 provides strong evidence for a Fermi liquid like normal state.

False color plot of the difference between the real and imaginary part of the optical conductivity for Ba122.

False color plot of the difference between the real and imaginary part of the optical conductivity for Ba122.

Transport experiments on crystals grown by Yingkai Huang showed that the resistivity of carefully annealed single crystals decreased, while the critical temperature increased significantly. This motivated us to investigate the optical properties of as-grown and annealed single crystals, which could provide important clues to the changes induced by annealing. Such information is usually hidden in the optical scattering rate and mass enhancement that can be extracted from the optical conductivity. Such an analysis is however complicated for materials where low energy optical transitions between different bands are present (such as is the case for the iron-pnictides). In the paper we point out this difficulty and provide a new method that circumvents this problem. By comparing the experimental results with detailed calculations we conclude that the normal state has all the hallmarks of a weakly correlated Fermi liquid. More information can be found under Highlights.


PhD position: optical spectroscopy below the diffraction limit

We are looking for a highly-motivated student who will be the driving force behind a collaboration between the optical spectroscopy lab at the University of Amsterdam and the EUV targets group at the Advanced Research Centre for Nanolithography (ARCNL).

The optical spectroscopy lab, supervised by Dr. Erik van Heumen, is part of the Quantum Matter Amsterdam (QMA) cluster and focusses on spectroscopy of topological and strongly correlated electron materials. The EUV targets group, supervised by Prof. Dr. Paul Planken, is part of ARCNL and focusses on the interaction between intense laser light and metals.
More information on our research can be found on the QMA or EUV targets webpages.

The project will focus on determining nanoscale resolved optical properties of metals, electronic devices and thin films using near-field optical techniques. You will take the lead in the further development of the near-field optics lab and will develop expertise in turning raw materials, single crystals grown in the QMA group, into field effect devices that can be used to tune their electronic properties. The aim of your work will be to uncover the dielectric and other physical properties of nanometer-sized metal particles for terahertz emission applications, and the impact of confined geometries on the electronic structure of topological and correlated materials. You will use this knowledge to develop new methods to enhance correlations or tune between topological phases of matter.

You hold a MSc. in (theoretical) physics or physical chemistry and are requested to motivate why you apply for the position and to supply a C.V. Applicants with a degree in chemistry are also requested to detail their affinity with condensed matter physics and motivation for pursuing a Ph.D in experimental physics.

Other skills/experiences/documents that would benefit your application are:
• Previous laboratory experience using some form of spectroscopy.
• Working knowledge of a programming language (phyton, C++ or equivalent).
• A background in theoretical condensed matter physics.
• Excellent English oral and written communication skills.
• Scientific publications and/or a reference letter from your MSc. thesis advisor.

For informal enquiries about the position please contact Erik van Heumen.

Applications must include:

  • a curriculum vitae.
  • a motivation letter that explains why you have chosen to apply for this specific position with a statement of your research experience and interests and how these relate to this project.
  • Title and summary of your Master thesis.

Please submit your application electronically. Applications will be processed on a rolling basis, and the position will remain open until a suitable candidate has been identified. (Formal closing date: 15 October 2017).

No agencies please

Relevant references:
• Huth, F. et al. Nano-FTIR Absorption Spectroscopy of Molecular Fingerprints at 20 nm Spatial Resolution. Nano Letters 12, 3973 (2012).
• McLeod, A.S., E. van Heumen et al. Nanotextured phase coexistence in the correlated insulator V2O3. Nature Phys. 13, 80 (2017)

Postdoc position available: QI for QM

The optical spectroscopy lab has a vacancy for a postdoctoral scholar for the duration of 2 years, working in close collaboration with the newly established QuSoft research center for quantum software.

The successful applicant will become a member of the QuSoft research community. The fellow is expected to become the driving force behind a new research effort that will explore the application of concepts from quantum information theory to the experimental investigation of highly entangled states appearing in strongly correlated electron systems. He or she is expected to benefit from close collaborations with researchers from a variety of disciplines (computational physics, mathematics and information science).

Scientific background

One of the most exciting insights that came with the advent of quantum computing is the notion that many-body electronic states in solids, bear a resemblance to the highly entangled quantum states of multi-qubit systems. This insight opened the door for the condensed matter community to adapt some of the concepts and language developed in the context of Quantum Information science. Mostly this cross-fertilization has led to new mathematical tools that enable progress in the study of correlated electron systems. Apart from a few isolated examples, what is missing are examples of practical implications of these ideas in a 'solid matter' realization. One example of such a new concept, waiting for transfer, is the discovery of a connection between the quantum Fisher information and the dynamic susceptibility (imaginary part of a thermal response function). The latter is experimentally readily accessible and thus provides a new tool to assess entanglement in many-body states. The quantum Fisher information is expected to become universal near quantum phase transitions, signaling a divergent multi-partite entanglement [1].


For informal enquiries about the positions please contact:

Applications should be sent to:

To enable us to process your application immediately, please quote vacancy number 16-411 and the position you are applying for in the subject-line.

Applications must include (in a single PDF attachment):

  • a curriculum vitae;
  • a motivation letter that explains why you have chosen to apply for this specific position with a statement of your research experience and interests and how these relate to this project;
  • a list of publications;
  • title and summary of your PhD thesis;
  • (URLs pointing to) your top two publications.

Applications will be processed on a rolling basis, the position will remain open until a suitable candidate has been identified. Formal closing date is 29 October 2016, to be extended if no suitable candidate has been found.

[1] Hauke, P., Heyl, M., Tagliacozzo, L. & Zoller, P. Measuring multipartite entanglement through dynamic susceptibilities. Nature Phys., DOI: 10.1038/NPHYS3700 (2016)

Quantum information for quantum matter

The optical spectroscopy group will be hosting a post-doctoral fellow to explore new links between Quantum Information theory and experimental probes of highly entangled many-particle ground states.

QuSoft, the new Dutch research center for quantum software, has awarded two junior research positions. After an internal call, the proposals were evaluated by an expert jury who selected the best two proposals.

The optical spectroscopy group will be hosting a post-doctoral fellow to explore new links between Quantum Information theory and experimental probes of highly entangled many-particle ground states. An important goal is to apply recent ideas from quantum information to current experimental problems in research on quantum matter, for instance by the experimental verification of a proposed relation between quantum Fisher information and optical properties of quantum matter.

QuSoft was recently launched by CWI, UvA and VU. Its mission is to develop new protocols, algorithms and applications that can be run on small and medium-sized prototypes of a quantum computer.

The position will be filled as soon as possible, keep posted for the advert.

Alona receives ICAM Junior travel award.

Alona Tytarenko is one of the recipients of an ICAM Junior travel award that enables her to attend the Condensed Matter Physics in the City 2015 meeting in London from June 22 - July 3rd.

alona tytarenko

The CMP in the City meeting is an annual meeting on the topic of strongly correlated quantum many body systems. This year the topics Topological, Spin and Orbital entanglement dominate the program. The ICAM Junior travel award also provides Alona with the opportunity to present her first talk at an international meeting. The topic of her talk will of course be the observation of a Fermi liquid like groundstate that we uncovered in our first publication

EXS2014: first optical lab results presented

We presented the first experimental results coming out of the optics lab at the EXS 2014 workshop.

The "2014 workshop on probing and understanding exotic superconductors" at the Abdus Salam ICTP in Trieste, Italy was held from 27 October to 31 October. Erik presented a contributed talk and Alona a poster. We are working hard on a first draft of these results, so stay tuned for the paper.  In the mean time you can get an idea of what the paper will be about by watching Alona's flash presentation of her poster.

Alona Tytarenko joined the group

Alona Tytarenko has joined the van Heumen lab. Alona's research will focus on optical spectroscopy of iron-pnictide superconductors and topological insulators.

alona tytarenkoAlona obtained her Master degree at the Taras Shevchenko National University in Kiev (Ukraine) in 2011. She defended her thesis “Photoacoustic transformation in composite systems based on porous silicon” with the mark “excellent”. After her thesis she worked on photoacoustic transformations in porous materials as a physics engineer in the same institute. Her work there resulted in publications in Mat. Lett. and Micropor. Mesopor Mat.
In January 2014 she joined the optical spectroscopy lab as PhD student focussing on the investigation of optical properties of superconducting and topological materials.

New tenure trackers at the IoP

Katharina Dohnalova and Erik van Heumen have joined the IoP as tenure trackers.

Katharina Dohnalova has joined the IoP as macGillavry fellow and will focus her research on silicon Quantum dots. Erik van Heumen has started a new group and will focus on investigating quantum materials using optical spectroscopy techniques.