scientist preparing magnetic maesurements

Latest news from Research Team "Synchrotron Methods"

compiled by Sergey Borisenko

Very First 3D Fermi Surface Map

To our best knowledge, this is the very first 3D Fermi surface map measured by ARPES in the full Brillouin zone along kz. It consists of approximately 100 conventional Fermi surface maps, each taken within a minute. FeSuMa makes it possible to perform a 3D-mapping within a very short time interval and with very high resolution. In combination with various light sources including synchrotron radiation our methodology and instrumentation offer new opportunities for high-resolution ARPES in physical and life sciences.

Figure: Comparison of the experimental and theoretical 3D Fermi surfaces of TiTe2

Sergey Borisenko, Bulletin of the American Physical Society, 2021

Spin Texture in One Minute

Having the possibility to map the Fermi surface almost instantaneously, we applied FeSuMa to detect the circular dichroism in topological surface states of Bi2Te3. This is another "one-parameter" experiment. The result is shown in the right part of the slide. First, the map is recorded using the righ-hand cricularly polarized light, then the light of opposite helicity is used. The difference spectrum clearly shows the dichroism caused by the spin-momentum locking at the surface of Bi2Te3.

Figure: Circular dichroism measured by FeSuMa.

Sergey Borisenko, Bulletin of the American Physical Society, 2021

Nematic Superconductivity

We never paid attention to the possible changes of the Fermi surface in LiFeAs with temperature earlier. A closer inspection also resulted in the observation of clearly C2-gap structure. Iin this paper we demonstrate that even the superconducting state in LiFeAs is a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations are hardly visible above superconducting Tc. Our results demonstrate the realization of the phenomenon of superconductivity-induced nematicity in IBSs, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and s−dmixing in the gap function.

Figure: Fermi surface of LiFeAs and its distortion in the superconductong state.

Yevhen Kushnirenko et al. Phys. Rev. B 102, 184502 (2020)

Nematic Superconductivity more precisely

Using the advantages of FeSuMa, we have carried out the experiments focusing on the smallest Fermi surfaces of LiFeAs. Laser radiation (6 eV) allows one to make it with the higher precision since the momentum resolution is better. We have recorded the Fermi surface twice, above the critical temperature and below it. It was important to keep only one parameter changing - the temperature. The rest - position of the sample, and other experimental conditions were kept essentially the same. Our results clearly show the shrinking of the Fermi surface along ky-direction upon entering the superconducting state, thus confirming our previous conclusions.

Figure: Deformation of the Fermi surface in LiFeAs and energy resolution of FeSuMa.

Sergey Borisenko, Bulletin of the American Physical Society, 2021

FeSuMa Collects First Laser Data

We are very happy that FeSuMa, designed by Fermiologics, is now able to detect very slow photoelectrons emitted from the surface by laser. We use 6 eV photons to study famous Bi2Te3. The Fermi surface, here the momentum distribution integrated within ~25 meV, can be seen in real time on the screen of PC.

Video: Fermi surface of the topological surface states in Bi2Te3.

Fermi Surface of BiTeI

BiTeI is widely known as a semiconductor which exhibits a giant Rashba splitting in the bulk. Although the material has been studied multiple times by different ARPES groups, there is no concensus as to which extent the electronic structure can be described in details. Particular uncertainty referes to which states should be considered as surface ones and which originate from the bulk. Our new measurements indicate that the situation can be actually dramatically different and the material itself should belong to a different class.

Figure: Comparison of the bulk Fermi surfaces obtained experimentally with the band structure calculations.

Sergey Borisenko, Talk at Cluster Retreat 2021, ct.QMAT

Quantum Well States?

Additional electronic states appearing in ARPES spectra  from canonical topological insulators Bi2Se3 and Bi2Te3 sparked  heated debates in the community. The states, looking alike, emerge from the conduction band continuum with the time. Now we see them directly using FeSuMa and can track their behaviour as a function of time and position on the sample more precisely. Our experiments shed new light on the origin of these states.

Figure: Quantum Well States in Bi2Te3.

Sergey Borisenko, Talk at Cluster Retreat 2021, ct.QMAT

Sixfold Fermion

Another successful example of the collaboration within the IFW: theory - synthesis - ARPES. Here we show that the cubic compound PtBi2, is a topological semimetal hosting a sixfold band touching point in close proximity to the Fermi level. Using angle-resolved photoemission spectroscopy, we map the bandstructure of the system, which is in good agreement with results from density functional theory. Further, by employing a low energy effective Hamiltonian valid close to the crossing point, we study the effect of a magnetic field on the sixfold fermion. The latter splits into a total of twenty Weyl cones for a Zeeman field oriented in the diagonal, [111] direction. Our results mark cubic PtBi2, as an ideal candidate to study the transport properties of gapless topological systems beyond Dirac and Weyl semimetals.

Figure: comparison with theory allows one to locate the topological elements.

Setti Thirupathaiah et al. SciPost Phys. 10, 004 (2021)

Planckian Dissipation

Another paper is published after a very long review process. This is a collaboration with Johnpierre Paglione's group. Quantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. We report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe1/3Co1/3Ni1/3)2As2, indicative of quantum criticality at zero temperature and applied magnetic field.

Figure: Fermi surface map, momentum-energy cuts and quantum critical scattering rate

Yasuyuki Nakajima et al. Communications Physics 3, 181 (2020)

Trivial Topology

A common wisdom is that many iron-based superconductors host topological surface states. In this paper we demonstrate, that the situation is not that trivial. We use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe1−xSex (x = 0.45) is a superconducting 3D Dirac semimetal hosting type-I and type-II Dirac points but that its electronic structure remains topologically trivial. We show that the inverted band gap in FeTe1−xSex can possibly be realized by further increase of Te content, but strong correlations reduce it to a sub-meV size, making the experimental detection of this gap and corresponding topological surface states very challenging, not to mention exact matching with the Fermi level. On the other hand, the pdand dd interactions are responsible for the formation of extremely flat band at the Fermi level pointing to its intimate relation with the mechanism of high-Tc superconductivity in IBS.

Figure: Key 3D-dispersion and summary of our results

Sergey Borisenko et al. npj Quantum Materials 5, 67 (2020) 

Turning Charge-Density Waves into Cooper Pairs

Here we collaborated with the group of Kee Hoon Kim. The idea was to see whether the superconductivity can be enhanced by making  CDW weaker. We demonstrated that by tuning the energy position of the van Hove singularity in Pd-doped 2H-TaSe2, one is able to suppress CDW and enhance superconductivity by more than an order of magnitude. We argued, again :), that it is particular fermiology of the material that is responsible for each phenomenon, thus explaining their persistent proximity as phases.

Figure: Sketch of the electronic structure of 2H-TaSe2, 2H-Pd0.08TaSe2, and 2H-NbSe2

Alla Chikina et al. npj Quantum Materials 5, 22 (2020) 

Non-Magnetic Half-Metals

This is another paper resulted from beautiful crystals of Bob Cava. Non‐magnetic half‐metals are a new class of materials that are metallic only for one spin direction. In contrast to conventional half‐metals, this spin direction is not fixed in space and is always perpendicular to the momentum of an electron. IrBiSe is one of such materials. Using angle‐resolved photoemission spectroscopy and band structure calculations we found a record‐high Dresselhaus spin‐orbit splitting, fully spin‐polarized remnant Fermi surfaces and a chiral 3D spin‐texture, all with no magnetism present.  Promising applications include using IrBiSe as a source of spin‐polarized electrons, and lightly doped IrBiSe is expected to generate electric‐field‐controlled spin‐polarized currents, free from back scattering, and could host triplet superconductivity.

Figure: Cover of RPL with our results. 

Zhnoghao Liu et al. physica status solidi (RRL)–Rapid Research Letters 14, 1900684 (2020)

First Magnetic Weyl Semimetal

This paper reports the spectroscopic evidence for experimental realization of the very first magnetic Weyl semimetal. Curiously, it took more than four years to publish it. The decisive evidence for canting in the candidate material came only recently from magentooptical studies. This is an example of how the novel exotic topological materials can be discovered by ARPES.

Figure: The way to Weyl.

Sergey Borisenko et al. Nature Communications 10, 3424 (2019)