P. Dziawa

Topological crystalline insulator states in Pb1−xSnxSe

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.

Spin-glass behavior in Ni-doped La1.85Sr0.15CuO4

The dynamic and static magnetic properties of La1:85Sr0:15Cu1-yNiyO4 with Ni concentration up to y=0.63 are reported. All the features that characterize the spin-glass (SG) behavior are found: bifurcation of the dc susceptibility chi vs temperature curve, a peak in the zero-field cooling branch of this curve accompanied by a step in the imaginary part of the ac susceptibility chi_ac, frequency dependence of the peak position in the real part of chi_ac and scaling behavior. The decay of remnant magnetization is described by a stretched-exponential function. The characteristic time from the critical slowing-down formula that governs the dynamics of the system suggests the existence of the spin clusters. The strongest interactions between the fluctuating entities are at y equal to the charge carriers concentration in the system. The SG transition temperature decreases linearly with decreasing y for y<0.30 and extrapolates to 0 K at y=0 what means that Ni display a magnetic character in the surrounding Cu-O network starting from the smallest concentration y. The static critical exponents characterizing the scaling behavior of the nonlinear part of chi lie between those typical for three dimensional Ising-like and Heisenberg-like systems

Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se

We study the nature of (001) surface states in Pb0.73Sn0.27Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above th ...

Observation of topological crystalline insulator surface states on (111)-oriented Pb1-xSnxSe films

We present angle-resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb1-xSnxSe, a three-dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at (Gamma) over bar and (M) over bar in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality, and alternative surface orientations in (Pb,Sn)Se solid solutions.

Robust spin-polarized midgap states at step edges of topological crystalline insulators

An edge that is hard to get rid of A distinguishing characteristic of topological insulators (TIs) is that they have conducting states on their boundary—a surface for a three-dimensional (3D) TI or a line edge for a 2D TI. Sessi et al. used scanning tunneling spectroscopy to discover unusual 1D states in a 3D crystalline TI. The states appeared on the edge of a particular kind of step in the crystal and survived large magnetic fields and increased temperatures. This robustness bodes well for the potential use of these states in practical applications. Science, this issue p. 1269 Scanning tunneling spectroscopy is used to uncover a one-dimensional protected state in (Pb,Sn)Se. Topological crystalline insulators are materials in which the crystalline symmetry leads to topologically protected surface states with a chiral spin texture, rendering them potential candidates for spintronics applications. Using scanning tunneling spectroscopy, we uncover the existence of one-dimensional (1D) midgap states at odd-atomic surface step edges of the three-dimensional topological crystalline insulator (Pb,Sn)Se. A minimal toy model and realistic tight-binding calculations identify them as spin-polarized flat bands connecting two Dirac points. This nontrivial origin provides the 1D midgap states with inherent stability and protects them from backscattering. We experimentally show that this stability results in a striking robustness to defects, strong magnetic fields, and elevated temperature.

Direct observation and temperature control of the surface Dirac gap in a topological crystalline insulator

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.

Band inversion and the topological phase transition in (Pb,Sn)Se

The recent discovery of a topological phase transition in IV-VI narrow-gap semiconductors has revitalized the decades-old interest in the bulk band inversion occurring in these materials. Here we systematically study the (001) surface states of Pb1-xSnxSe mixed crystals by means of angle-resolved photoelectron spectroscopy in the parameter space 0 = T >= 9 K. Using the surface-state observations, we monitor directly the topological phase transition in this solid solution and gain valuable information on the evolution of the underlying fundamental band gap of the system. In contrast to common model expectations, the band-gap evolution appears to be nonlinear as a function of the studied parameters, resulting in the measuring of a discontinuous band-inversion process. This finding signifies that the anticipated gapless bulk state is in fact not a stable configuration and that the topological phase transition therefore exhibits features akin to a first-order transition.

Ferromagnetic Transition in Ge_{1-x}Mn_{x}Te Layers

Ferromagnetic transition temperature in thin layers of diluted magnetic (semimagnetic) semiconductor Ge1−xMnxTe was studied experimentally by SQUID magnetometry method and analyzed theoretically for a model Ising-type diluted magnetic system with Ruderman–Kittel–Kasuya–Yosida indirect exchange interaction. The key features of the experimentally observed dependence of the Curie temperature on Mn content (x ≤ 0.12) and conducting hole concentration p = (1–10)× 10 cm−3 were reproduced theoretically for realistic valence band and crystal lattice parameters of p-Ge1−xMnxTe taking into account short carrier mean free path encountered in this material and Ruderman–Kittel–Kasuya–Yosida mechanism with both delta-like and diffused character of spatial dependence of the exchange coupling between magnetic ions and free carriers.

Novel ZnO/MgO/Fe2O3 composite optomagnetic nanoparticles.

A facile sol-gel synthesis of novel ZnO/MgO/Fe2O3 nanoparticles (NPs) is reported and their performance is compared to that of ZnO/MgO. Powder x-ray diffraction (XRD) patterns reveal the crystal structure of the prepared samples. The average particle size of the sample was found to be 4.8 nm. The optical properties were determined by UV-vis absorption and fluorescence measurements. The NPs are stable in biologically relevant solutions (phosphate buffered saline (PBS), 20 mM, pH = 7.0) contrary to ZnO/MgO NPs which degrade in the presence of inorganic phosphate. Superparamagnetic properties were determined with a superconducting quantum interference device (SQUID). Biocompatible and stable in PBS ZnO/MgO/Fe2O3 core/shell composite nanocrystals show luminescent and magnetic properties confined to a single NP at room temperature (19-24 ° C), which may render the material to be potentially useful for biomedical applications.

Epitaxial Growth and Characterization of PbGeEuTe Layers

The structural and electrical properties of Pb1−yGeyTe and Pb1−x−yGeyEuxTe (0≤y≤0.4 and x≤0.05) monocrystalline layers grown by molecular beam epitaxy technique on BaF2 (111) substrate were studied by X-ray diffraction, Hall effect, and electrical conductivity measurements. Based on the temperature dependence of the lattice parameter the structural (ferroelectric) transition temperature was found in the temperature range before 100 to 250 K in layers with varying Ge and Eu content. Electrical measurements indicates that incorporation of Eu ions in the PbGeTe crystal matrix decreases the electrical conductivity in p-type PbGeEuTe layers by 1–2 orders of magnitude.