ABSTRACT This work investigates SnTe thin films using microwave absorption as a probe for the presence of Dirac fermions. Shubnikov–de Haas oscillations observed in the microwave absorption signal were analyzed for both uncapped and capped samples, where a BaF2 cap layer was employed to protect the topological surface states from degradation. Unexpectedly, signatures of Dirac fermions were also detected in the uncapped sample, despite previous magnetotransport measurements showing no evidence of their presence. This finding provides strong evidence that the topological surface states are robust against deterioration due to atmospheric exposure and that microwave absorption measurements can reveal these states even when they are not accessible through transport measurements. Furthermore, Zeeman splitting was observed in the fast Fourier transform of the oscillations, indicating the presence of low‐mobility channels that had previously been detected only at higher magnetic fields.

Chiral phonons carry angular momentum and respond to magnetic fields, exhibiting peculiar properties such as the phonon Zeeman effect. In recent experimental studies, we reported the dependence of the phonon magnetic moment on electronic topology. However, the tuning of phonon magnetic properties via free carrier density remains unexplored. To provide a material platform for that purpose, we now demonstrate the emergence of distortive polar order by controlling the carrier concentration in topological epilayers.

Polar metals, materials that exhibit both electric polarization and high conductivity, can also host topological phases. Because free carriers strongly suppress distortive polar order and change the Fermi level, controlling charge dynamics is crucial for simultaneously tuning ferroelectric and topological phases in the same material. Here, we explore the experimental conditions that enable access to these phases in bismuth-doped Pb_1-xSn_xTe epilayers. For samples in the topological phase at x = 0.5, we use terahertz time-domain spectroscopy to evaluate their complex permittivity as a function of temperature. We observe a non-monotonic variation in carrier concentration with bismuth doping, indicating a change in carrier type. By tracking the transverse optical phonon mode, we identify a ferroelectric phase transition when distortive polar order emerges below a critical temperature that depends on carrier concentration. We show that bismuth doping controls the metallicity-dependent order parameters in the softening and hardening phases. Our work demonstrates a tunable platform for engineering exotic states of matter that integrate metallicity, ferroelectricity and topology.

Lead telluride is an important thermoelectric material due to its large Seebeck coefficient combined with its unusually low thermal conductivity that is related to the strong anharmonicity of phonons in this material. Here, we have studied the resonant and nonperturbative coupling of transverse optical phonons in lead telluride with cavity photons inside small-mode-volume metallic metasurface cavities that have photonic modes with terahertz frequencies. We observed a giant vacuum Rabi splitting on the order of the bare phonon and cavity frequencies. Through terahertz time-domain spectroscopy experiments, we systematically studied the vacuum Rabi splitting as a function of sample thickness, temperature, and cavity length. Under the strongest light-matter coupling conditions, the strength of coupling exceeded the bare phonon and cavity frequencies, putting the system into the deep-strong coupling regime. These results demonstrate that this uniquely tunable platform is promising for realizing and understanding predicted cavity-vacuum-induced ferroelectric instabilities and exploring applications of light-matter coupling in the ultrastrong and deep-strong coupling regimes in quantum technology.

In this work, the transport properties of a sample grown by molecular beam epitaxy are investigated. The sample is composed of a 15 nm - thick bismuth telluride (Bi2Te3) epitaxial film deposited on a barium fluoride (BaF2) (111) substrate and capped with a 75 nm BaF2 cover layer, which aims to prevent the topological surface states (TSSs) deterioration. Electrical resistance temperature dependence shows a transition from metallic to insulating behavior at approximate to 20 K as the temperature decreases. A detailed analysis of this behavior indicates a transition of the dominant conduction channel from bulk to TSS. Magnetoresistance (MR) measurements are performed in the temperature range of 1.9-200 K, and for high magnetic fields (B > 3 T), linear MR is observed at temperatures below 30 K, which also suggests the presence of TSSs. At low fields and low temperatures, MR curves exhibit a weak antilocalization effect. The experimental data are analyzed using the Hikami-Larkin-Nagaoka (HLN) equation. The obtained parameters of the HLN equation indicate the presence of two conduction channels at low temperature, and at least one of them originates from TSSs. The implementation of the cap layer successfully protects the TSSs and keeps the good performance of the electrical signal in the samples.

In this work, we present the investigation of the magnetotransport properties of a capped SnTe film, grown by molecular beam epitaxy, using Shubnikov–de Haas oscillations for the detection of Dirac fermions. The cap layer used was a 10 nm thick Sn0.98Eu0.02Te film, which can also contribute to the transport such that it is mandatory to isolate its contribution from the electrical transport measured in the sample. To separate the contribution from both layers, photoconductivity measurements were performed. A detailed analysis of the Shubnikov–de Haas oscillations is carried out using theoretical expressions and building the Landau-level indexation. We found that Dirac fermions are detected in the SnTe layer, while the cap layer contributes with trivial fermions, protecting SnTe against deterioration due to exposure to the atmosphere.

This work presents the investigation of photoconductivity effect on CaF2 doped PbTe films, revealing a strong effect of the doping in the electrical and photoconductivity properties. For the electrical properties, Hall effect measurements were carried out under dark and illuminated conditions. We show that the photoconductivity amplitude suffers a huge increase for the doped samples at low temperatures while the persistent photoconductivity, commonly observed at room temperature for PbTe compounds, is suppressed. A simple classical Drude model for photoconductivity amplitude describes the observed effects.

This work presents the investigation of the photoconductivity effect in undoped and doped Pb0.5Sn0.5Te epitaxial films, with bismuth (Bi) atoms, at temperatures of 80 and 300 K. The results indicate that the samples show negative photoconductivity effect (NPC) and persistent photoconductivity (PPC). A detailed study was performed on Pb0.5Sn0.5Te sample doped with 0.15 % Bi, which presented higher photoconductivity amplitude than the other samples, by performing Hall effect and photoconductivity measurements in the temperature range of 80-300 K under dark and illuminated conditions. Using Arrhenius model, trap activation energy was extracted and compared with energies found in literature. From the Hall measurement we found that the NPC effect observed is due to a decrease in the mobility when the sample is illuminated while the carrier concentrations are nearly unaltered. It was also found that Pb0.5Sn0.5Te:Bi presented photoconductivity response for a wide range of wavelengths, indicating that it is potentially interesting for application in optic sensor devices.

The recently demonstrated chiral modes of lattice motion carry angular momentum and therefore directly couple to magnetic fields. Notably, their magnetic moments are predicted to be strongly influenced by electronic contributions. Here, we have studied the magnetic response of transverse optical phonons in a set of Pb 1− x Sn x Te films, which is a topological crystalline insulator for x > 0.32 and has a ferroelectric transition at an x -dependent critical temperature. Polarization-dependent terahertz magnetospectroscopy measurements revealed Zeeman splittings and diamagnetic shifts, demonstrating a large phonon magnetic moment. Films in the topological phase exhibited phonon magnetic moment values that were larger than those in the topologically trivial samples by two orders of magnitude. Furthermore, the sign of the effective phonon g -factor was opposite in the two phases, a signature of the topological transition according to our model. These results strongly indicate the existence of interplay between the magnetic properties of chiral phonons and the topology of the electronic band structure.

The most relevant characteristic of a topological insulator material is the presence of edge/surface states that are protected by the bulk topology, and therefore, insensitive to nonmagnetic disorder. However, if such disorder is induced by magnetic atoms or the topological insulator is subjected to an external magnetic field, the time-reversal symmetry is expected to break down, affecting the robustness of the edge/surface states. In this work, europium (Eu)-doped bismuth telluride thin films were grown by molecular beam epitaxy in order to analyze the effect of a small fraction of atoms with magnetic properties on topologically protected surface states. For films with different Eu concentrations, morphological and electronic characterizations were carried out using atomic force microscopy, scanning tunnelling microscopy, and scanning tunnelling spectroscopy (STS) techniques. The results show that, regardless of the Eu concentration, the layered structure characteristic of the Bi2Te3 phase is maintained. However, large (>2%) Eu concentrations induce the appearance of protrusions and clusters on the surface of the films. The STS measurements show the presence of surface states for pure and low-content Eu:Bi2Te3. The suppression of surface states is indicated by STS spectra in regions with well-defined gaps for some locally limited regions of our samples with large concentration of Eu atoms. From density functional theory we are able to show that the Eu substitutional impurity at the Bi site is not the main mechanism responsible for the observed changes in the topological insulator band structure. Furthermore, the magnetic properties of europium are not the key factor dictating the different Bi2Te3 local surface electronic properties experimentally observed by STS, which are mostly affected by alloying and atom replacement, which induce a chemical modification of the surface potential.

We have studied the resonant and nonperturbative coupling of transverse optical phonons in lead telluride with photons in small-mode-volume terahertz cavities, observing a giant vacuum Rabi splitting on the order of the uncoupled phonon and cavity frequencies. Our terahertz time-domain spectroscopy experimental data, systematically collected as a function of sample thickness, temperature, and cavity length, can be well reproduced by our electromagnetic simulations. These results demonstrate that this uniquely tunable platform is promising for realizing and understanding predicted cavity-vacuum-induced ferroelectric instabilities, as well as for exploring applications of light-matter coupling in the ultra-and deep-strong coupling regimes in quantum technology.

We have investigated the resonant coupling of photons with TO phonons in lead telluride in small-mode-volume terahertz cavities, observing a giant vacuum Rabi splitting on the order of the bare cavity–phonon frequency.

The exceptional thermoelectric properties of PbTe are believed to be associated with the incipient ferroelectricity of this material, which is caused by strong electron–phonon coupling that connects phononic and electronic dynamics. Here, we have used terahertz time-domain spectroscopy measurements to generate complex permittivity spectra for a set of epitaxially grown PbTe thin films with thicknesses between 100 nm and 500 nm at temperatures from 10 K to 300 K. Using a Drude–Lorentz model, we retrieved the physical parameters of both the phononic and electronic contributions to the THz permittivity. We observed a strong decrease, or softening, of the transverse optical phonon mode frequency with decreasing temperature, determining a thickness-independent negative ferroelectric-transition critical temperature, while we found a thickness-dependent anharmonic phonon decay lifetime. The electronic contribution to the permittivity was larger in thinner films, and both the carrier density and mobility increased with decreasing temperature in all films. Finally, we detected a thickness-dependent longitudinal optical phonon mode frequency, indicating the presence of plasmon–phonon coupling.

The formation of Janus-like particles of Pb-Sn during ion-plasma treatment of the surface of lead-tin telluride films was found. Pb0.6Sn0.4Te films 2 mu m thick were grown on (111) BaF2 substrates by molecular beam epitaxy. The ion-plasma treatment of the samples was carried out in a high-density low-pressure radio frequency inductively coupled plasma at an ion energy of 75 eV and 25 eV. The duration of the sputtering process was 240 s. The evolution of the film surface morphology and the formation of Pb-Sn Janus particles with nano-and submicron sizes have been studied.

In this work, we performed magnetotransport measurements in an n-type single quantum well (QW) grown by molecular beam epitaxy and found that, under illumination, the QW presents metallic behavior and Shubnikov-de Haas oscillations at low temperatures. Under dark conditions, the QW is an insulator and no quantum oscillations are observed. Several peaks were identified in the fast Fourier transform curves and some were addressed to the longitudinal and oblique valleys that compound the multiple-valley Fermi surface of PbTe. By investigating the Berry phases of the oscillation, a phase of ∼0.5, characteristic of Dirac Fermions, was obtained, in accordance with recent theoretical predictions of transport via topological surface states in PbTe films.

We report here on the properties of topological crystalline insulator Pb0.5Sn0.5Te epitaxial films doped with bismuth at levels from 0% (undoped) to 0.15%. The undoped film exhibits a p-type character due to metal vacancies. As the doping level rises, the hole concentration reduces. At a level of 0.06%, the electrical character inverts to n-type and the electron density continues to increase for rising doping level up to 0.15%. This result demonstrates an effective extrinsic n-type doping of Pb0.5Sn0.5Te crystal with bismuth due to substitutional Bi atoms in metal sites. High-resolution x-ray diffraction and reciprocal space mapping show that fully relaxed high-quality films are obtained. A pristine (111) film surface is revealed after removal of the Te cover layer using a method combining Ar+ sputtering and thermal desorption. Angle-resolved photoemission spectroscopy (ARPES) data acquired at 30 K near the Γ¯ point of the undoped film surface show a parabolic-like dispersion of the bulk valence band close to the Fermi level. Now, the ARPES data for a sample doped with 0.1% of Bi reveal that the chemical potential is shifted by 40 meV upwards in the direction of the conduction band. The ARPES results also indicate that there might be a discrepancy between surface and bulk chemical potential in the doped sample. This divergence suggests that Te atoms diffuse into the surface during the thermal process to desorb the protective layer, inverting the surface to p-type.

Previously, we showed that at low temperatures and very low excitation powers, EuTe displays band-edge photoluminescence with a well-resolved phonon structure, the so-called MX0 band, which is associated with the recombination of giant spin polarons (SPs) of magnetic moments of several hundred Bohr magnetons. Here, we investigate the MX0 band both experimentally and by Monte Carlo simulations. Results indicate that the SPs are bound to defects, with a localization energy of 0.29 eV. The density of defects harboring the SPs is estimated to be 4.3×1015 cm−3.

Magnetoresistance measurements were performed on a 30 nm-thick SnTe quantum well (QW) grown by molecular beam epitaxy on the BaF2 substrate in the temperature range of 1.9–50 K. The weak antilocalization (WAL) effect was observed at low temperatures and low magnetic fields as a result of the strong spin–orbit coupling present in the QW. Using the Hikami–Larkin–Nagaoka equation, we analyzed the experimental data and found that the WAL effect is not purely 2D but composed of 2D and 3D channels that exist within the QW structure. The spin–orbit and phase coherence mechanisms are also extracted, and a general view of the transport properties of the QW is also provided.

We have studied the magnetic response of transverse optical phonons in Pb$_{1-x}$Sn$_{x}$Te films. Polarization-dependent terahertz magnetospectroscopy measurements revealed Zeeman splittings and diamagnetic shifts, demonstrating that these phonon modes become chiral in magnetic fields. Films in the topological crystalline insulator phase ($x > 0.32$) exhibited magnetic moment values that are larger than those for topologically trivial films ($x < 0.32$) by two orders of magnitude. Furthermore, the sign of the effective $g$-factor was opposite in the two phases, which can be explained by our theoretical model. These results strongly indicate the existence of interplay between the magnetic properties of chiral phonons and the topology of electronic band structure.

PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25 T, the phonon mode splits into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies display Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are the result of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Thus, our Letter reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.