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Since the advent of topological insulators hosting Dirac surface states, efforts have been made to gap these states in a controllable way. A new route to accomplish this was opened up by the discovery of topological crystalline insulators where the topological states are protected by crystal symmetries and thus prone to gap formation by structural changes of the lattice. Here we show a temperature-driven gap opening in Dirac surface states within the topological crystalline insulator phase in (Pb,Sn)Se. By using angle-resolved photoelectron spectroscopy, the gap formation and mass acquisition is studied as a function of composition and temperature. The resulting observations lead to the addition of a temperature- and composition-dependent boundary between massless and massive Dirac states in the topological phase diagram for (Pb,Sn)Se (001). Overall, our results experimentally establish the possibility to tune between massless and massive topological states on the surface of a topological system. The opening of a Dirac point gap in topologically non-trivial materials is key to potential applications. Here, the authors use photoelectron spectroscopy to study gap formation and carrier mass acquisition in a topological crystalline insulator as a function of composition and temperature.

Topological crystalline insulators are a novel state of matter in which the topological features of the electronic structure have been predicted to originate from crystal symmetries. Now an experimental realization of a topological crystalline insulator is reported, in the form of Pb 1− x Sn x Se. Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection 1 , 2 . In this study we show that the narrow-gap semiconductor Pb 1− x Sn x Se is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.

We present the studies of structural, magnetotransport, and magnetic properties of Ge1−xEuxTe bulk crystals with the chemical composition, x, changing from 0.008 to 0.025. For the samples with x > 0.015 the sample synthesis leads to formation of Ge1−xEuxTe spinodal decompositions with a broad range of chemical contents. The presence of Ge1−xEuxTe spinodal decompositions is responsible for the antiferromagnetic order in our samples with x > 0.015. For the samples with x < 0.015 the structural characterization shows no evidence for clusters, the samples are paramagnetic, but the analysis of the results of magnetic measurements indicates deviations from the random distribution of Eu ions.

Topological nodal-line semimetals represent a unique class of materials with intriguing electronic structures and rich of symmetries, hosting electronic states with nontrivial topological properties. Among these, ZrAs\\(_2\\) stands out, characterized by its nodal lines in a momentum space, governed by nonsymmorphic symmetries. This study integrates angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations to explore the electronic states of ZrAs\\(_2\\). Our study provides experimental evidence of nonsymmorphic symmetry-protected band crossing and nodal lines in ZrAs\\(_2\\). In ARPES scans, we observed a distinctive surface and bulk states at different photon energies associated with nodal lines. Our results, supported by calculations based on DFT, unveil such impervious band crossing anchored at specific points in the Brillouin zone, with particular emphasis on the S point. Surface bands and bulk states near the crossing are elucidated through slab calculations, corroborating experimental findings. These findings enhance our understanding of the electronic structure of ZrAs\\(_2\\).

The paper presents the experimental results of the electronic band structure study of the clean PbGdTe surface and this surface additionally doped with gadolinium atoms. Gadolinium thin films (0.4 and 1 nm) were grown epitaxially on the PbGdTe substrate. After the second evaporation the sample was annealed. Heating of the sample covered by metal atoms led to the diffusion of the gadolinium atoms into the surface layer of the sample. After each stage of the sample treatment (surface cleaning, Gd evaporation and annealing) resonant photoemission spectra were acquired for the photon energy range of 130–152 eV, corresponding to the Fano-type Gd 4d-4f resonance. The emission from the Gd 4f shell of the atoms built into the surface layer of PbGdTe was revealed and its binding energy was determined as equal to 10.4 eV

We have performed electron transport and ARPES measurements on single crystals of transition metal dipnictide TaAs2 cleaved along the (\\(\\overline{2}\\) 0 1) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between the magnetic field and the [\\(\\overline{2}\\) 0 1] direction. The results indicate the elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved (\\(\\overline{2}\\) 0 1) surface and it was found that bulk states pockets at the constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level is increased, which is due to a small n-doping of the samples. This shifts the Fermi level closer to the Dirac point, allowing to investigate the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.

The possibility of inducing superconductivity in type-I Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by XRD, EDX, SEM, ARPES techniques and by electron transport measurements. The mobility spectrum of the carriers was determined. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces as a function of the DC bias were investigated. When the metals become superconducting, all three types of junctions show conductance increase, pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is satisfactorily described by modified Blonder-Tinkham-Klapwijk model. The absolute value of the conductance is much smaller than that for the bulk crystal, indicating that the transmission occurs through only a small part of the contact area. An opposite situation occurs in In-NbP junction, where the conductance at the peak reaches the bulk value indicating that almost whole contact area is transmitting and, additionally, a superconducting proximity phase is formed in the material. We interpret this as a result of indium diffusion into NbP, where the metal atoms penetrate the surface barrier and form very transparent superconductor-Weyl semimetal contact inside. However, further diffusion occurring already at room temperature leads to degradation of the effect, so it is observed only in the pristine structures. Despite of this, our observation directly demonstrates possibility of inducing superconductivity in a type-I Weyl semimetal.

We present the studies of structural, magnetotransport, and magnetic properties of Ge 1− x Eu x Te bulk crystals with the chemical composition, x , changing from 0.008 to 0.025. For the samples with x  > 0.015 the sample synthesis leads to formation of Ge 1− x Eu x Te spinodal decompositions with a broad range of chemical contents. The presence of Ge 1− x Eu x Te spinodal decompositions is responsible for the antiferromagnetic order in our samples with x  > 0.015. For the samples with x  < 0.015 the structural characterization shows no evidence for clusters, the samples are paramagnetic, but the analysis of the results of magnetic measurements indicates deviations from the random distribution of Eu ions.

Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In this study we show that the narrow-gap semiconductor Pb sub(1-x)Sn sub(x)Se is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.

Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In this study we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a TCI for x  =  0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.