We have performed electron transport and angle-resolved photo-emission spectroscopy (ARPES) measurements on single crystals of transition metal dipnictide TaAs2cleaved along the (2¯01) 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 magnetic field and the [2¯01] direction. The results indicate 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 (2¯01) surface and it was found that bulk states pockets at 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 (FL) is increased, which is due to a small n-doping of the samples. This shifts the FL closer to the Dirac point, allowing investigating 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.

HgTe quantum wells with a thickness of ∼7 nm may have a graphene-like band structure and have been recently proposed to be potential candidates for quantum Hall effect (QHE) resistance standards under the condition of operation in the fields above certain critical field B_c, above which the topological phase (with parasitic edge conduction) disappears. We present experimental studies of the magnetoresistance of different of HgTe quantum wells as a function temperature and magnetic field, determining the critical magnetic field B_c. We demonstrate that for QWs of specific width B_c becomes low enough to grant observation of remarkably wide QHE plateaus at the filling factor v=-1 (holes) in relaxed cryomagnetic conditions: while using commercial 0.82 T Neodymium permanent magnets and temperature of a few Kelvin provided by ^4He liquid system only. Band structure calculations allow us to explain qualitatively observed phenomena due to the interplay between light holes and heavy holes energy sub-bands (side maxima of the valence band). Our work clearly shows that the peculiar band structure properties of HgTe QWs with massless Dirac fermions make them an ideal platform for developing metrological devices with relaxed cryomagnetic conditions.

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.

1 International Research Centre CENTERA, Institute of High Pressure Physics, Polish Academy of Sciences, 01-142 Warsaw, Poland 2 Institute for Physics of Microstructures RAS, GSP-105, 603950, N. Novgorod, Russia Laboratoire Charles Coulomb (L2C), UMR CNRS 5221, University of Montpellier, F-34095 Montpellier, France Institute of Physics PAS, al. Lotników 32/46, PL 02-668 Warsaw, Poland International Research Centre MagTop, al. Lotników 32/46, PL 02-668 Warsaw, Poland Rzhanov Institute of Semiconductor Physics SB of RAS, 630090 Novosibirsk, Russia

Experimental measurements of low‐temperature electron transport across Si nanogratings with 200 nm period are reported. The structure is fabricated of silicon on insulator layer using laser interference lithography followed by reactive ion etching. For transport measurements, macroscopic Hall bar formed in the patterned layer is used. The main result is negative magnetoresistance observed for temperatures lower than 60 K and not saturating in magnetic fields up to 9 Tesla. It is interpreted in terms of weak localization suppression and fit the magnetoresistance curves by both the two‐dimensional and three‐dimensional theoretical models. Surprisingly, both models describe satisfactorily the data and thus the problem of dimensionality remains unsettled. However, obtained values of the phase coherence lengths are significantly smaller than both the nanograting period and layer thickness, indicating that the dominant scattering mechanism is not result of the nanostructure geometry.

HgTe quantum wells (QWs) are two-dimensional semiconductor systems that change their properties at the critical thickness d c , corresponding to the band inversion and topological phase transition. The motivation of this work was to study magnetotransport properties of HgTe QWs with thickness approaching d c , and examine them as potential candidates for quantum Hall effect (QHE) resistance standards. We show that in the case of d > d c (inverted QWs), the quantization is influenced by coexistence of topological helical edge states and QHE chiral states. However, at d ≈ d c , where QW states exhibit a graphene-like band structure, an accurate Hall resistance quantization in low magnetic fields ( B ≤ 1.4 T) and at relatively high temperatures ( T ≥ 1.3 K) may be achieved. We observe wider and more robust quantized QHE plateaus for holes, which suggests—in accordance with the “charge reservoir” model—a pinning of the Fermi level in the valence band region. Our analysis exhibits advantages and drawbacks of HgTe QWs for quantum metrology applications, as compared to graphene and GaAs counterparts.

Weak antilocalization measurements has become a standard tool for studying quantum coherent transport in topological materials. It is often used to extract information about number of conducting channels and dephasing length of topological surface states. We study thin films of prototypical topological crystalline insulator SnTe. To access microscopic characteristic of these states we employ a model developed by Tkachov and Hankiewicz, [Physical Review B 84, 035444]. Using this model the spatial decay of the topological states is obtained from measurements of quantum corrections to the conductivity in perpendicular and parallel configurations of the magnetic field. Within this model we find interaction between two topological boundaries which results in scaling of the spatial decay with the film thickness. We attribute this behavior to bulk reservoir which mediates interactions by scattering events without phase breaking of topological carriers.

We investigate experimentally transport in gated microsctructures containing a band-inverted HgTe/Hg${}_{0.3}$Cd${}_{0.7}$Te quantum well. Measurements of nonlocal resistances using many contacts prove that in the depletion regime the current is carried by the edge channels, as expected for a two-dimensional topological insulator. However, high and nonquantized values of channel resistances show that the topological protection length (i.e., the distance on which the carriers in helical edge channels propagate without backscattering) is much shorter than the channel length, which is $\ensuremath{\sim}$100 $\ensuremath{\mu}$m. The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasiperiodic resistance fluctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled.

We report on experiments allowing to set an upper limit on the magnitude of the spin Hall effect and the conductance by edge channels in quantum wells of PbTe embedded between PbEuTe barriers. We reexamine previous data obtained for epitaxial microstructures of n-type PbSe and PbTe, in which pronounced nonlocal effects and reproducible magnetoresistance oscillations were found. Here we show that these effects are brought about by a quasi-periodic network of threading dislocations adjacent to the BaF$_2$ substrate, which give rise to a p-type interfacial layer and an associated parasitic parallel conductance. We then present results of transport measurements on microstructures of modulation doped PbTe/(Pb,Eu)Te:Bi heterostructures for which the influence of parasitic parallel conductance is minimized, and for which quantum Hall transport had been observed, on similar samples, previously. These structures are of H-shaped geometry and they are patterned of 12 nm thick strained PbTe quantum wells embedded between Pb$_{0.92}$Eu$_{0.08}$Te barriers. The structures have different lateral sizes corresponding to both diffusive and ballistic electron transport in non-equivalent L valleys. For these structures no nonlocal resistance is detected confirming that PbTe is a trivial insulator. The magnitude of spin Hall angle gamma is estimated to be smaller than 0.02 for PbTe/PbEuTe microstructures in the diffusive regime.

We report on a magnetospectroscopy study of a set of four HgTe quantum wells of different thickness. In quantizing magnetic fields, intraband and interband transitions have been observed. The obtained results are compared with the allowed transition energies calculated using the 8 × 8 Kane model.

Recently, a new class of materials, so-called topological insulators, has emerged. These are systems characterized by the inversion of the electronic band structure and also by a certain strength of the spin-orbit interaction. HgTe/CdxHg1-xTe quantum wells represent a prominent example. They can change to the topological insulator phase from the conventional insulator phase when the thickness of the quantum well is increased over the critical thickness d(c) = 6.3 nm. Here, we report on a far-infrared magnetospectroscopy study of a set of HgTe/CdxHg1-xTe quantum wells with different thicknesses from below to above the critical value d(c). In quantizing magnetic fields up to 16 T, both intraband and interband transitions have been clearly observed. In the widest quantum well with inverted band structure, we confirm the avoided crossing of the zero-mode Landau levels observed earlier in similar structures. In both noninverted quantum wells close to the critical thickness, we report unambiguously on the square root dependence of the transition energy on the magnetic field, as expected in the single-particle model of massless Dirac fermions. The obtained results are compared with the allowed transition energies between Landau levels in the valence and conduction bands calculated using the 8 x 8 Kane model.

We study the superconductor-insulator transition in Si/Nb/Si trilayers, in which the thickness of Si is fixed at 10 nm, and the nominal thickness of Nb changes in the range between d = 20 nm down to d = 0.3 nm. The transmission electron microscopy indicates the formation of the mixed Nb-Si layer for small d. Both the thickness-induced, and the magnetic-field induced superconductor-insulator transition is observed. The crossing point of the isotherms at the critical field B(c) decreases with decreasing d, and it is T-independent at temperatures below 300 mK. At larger fields the weak peak in magnetoresistance appears in some of the films.

We report on electron transport studies on superconductor/semiconductor hybrid structures of indium and n-type PbTe quantum wells embedded between Pb0.92Eu0.08Te barriers. In/PbTe contacts form by spontaneous alloying, which occurs already at room temperature. The alloyed phase penetrates deeply into the semiconductor and forms metallic contacts even in the presence of depletion layers at the semiconductor’s surface. Although the detailed structure of this phase is unknown, we observe that it exhibits a superconducting transition at temperatures below 7 K. This causes such substantial reduction in the contact resistances that they even become comparable to those predicted for ideal superconductor-normal conductor contacts. Our findings suggest that the new superconducting phase is result of Josephson coupling arising between tiny superconducting precipitates randomly distributed in the interface region. In analogy to the granular superconducting systems, the coupling would lead to the occurrence of the global superconductivity only when the Coulomb charging energies of the precipitates are sufficiently small. This condition is exceptionally well fulfilled in PbTe because of a huge static dielectric constant of this material, ε>1000.

The unexpected "0.7" plateau of conductance quantisation is usually observed for ballistic one-dimensional devices. In this work we study a quasi-ballistic quantum wire, for which the disorder-induced backscattering reduces the conductance quantisation steps. We find that the transmission probability resonances coexist with the anomalous plateau. The studies of these resonances as a function of the in-plane magnetic field and electron density point to the presence of spin polarisation at low carrier concentrations and constitute a method for the determination of the effective g-factor suitable for disordered quantum wires.

This article provides a review of our results on nanostructurization of lead telluride, PbTe. This IV–VI group narrow-gap semiconductor exhibits paraelectric behaviour leading to a huge dielectric constant ε>1000 at helium temperatures. Because the Coulomb potential fluctuations produced by charged defects are strongly suppressed in PbTe nanostructures, one can reach the quantum ballistic regime at significantly relaxed conditions in comparison with other systems. In particular, we observe precise zero-field conductance quantization in the wires made of modulation doped PbTe/PbEuTe quantum wells where the heavily doped layer is separated from the conducting channel only by a 2nm thick spacer layer. The second important property is the very large Zeeman splitting. It reaches 4meV/T. Accordingly, significant spin splitting of the conductance plateaux is observed already at fields below 1T. Therefore, the system is attractive for the construction of local spin filters. We show that the presence of metal layers does not impair the quantum ballistic properties. Furthermore, we have developed a new method of tuning the PbTe nanostructures, using laterally placed metallic electrodes. We have found that this method is more effective than previous schemes using used p–n junctions and it provides better stability of the nanostructures.

This is a review of our recent developments in the physics of lead telluride nanostructures. PbTe is a IV-VI narrow gap paraelectric semiconductor, characterized by he huge static dielectric constant epsilon > 1000 at helium temperatures. We nanostructurized this material by means of e-beam lithography and wet chemical etching of modulation doped PbTe/Pb1-xEuxTe quantum wells. Magnetoresistance measurements performed on the nanostructures revealed a number of magnetosize effects, confirming a ballistic motion of the carriers. The most important observation is that the conductance of narrow constrictions shows a precise zero-field quantization in 2e(2)/h units, despite a significant amount of charged defects in the vicinity of the conducting channel. This unusual result is a consequence of a strong suppression of the Coulomb potential fluctuations in PbTe, an effect confirmed by numerical simulations. Furthermore, the orbital degeneracy of electron waveguide modes can be controlled by the width of PbTe/Pb1-xEuxTe quantum wells, so that unusual sequences of plateau conductance can be observed. Finally, conductance measurements in a nonlinear regime allowed for an estimation of the energy spacing between the one-dimensional subbands.

The present article is a review of our experimental work performed on PbTe nanostructures. The uniqueness of lead telluride lies in a combination of excellent semiconducting properties, such as high electron mobility and tuneable carrier concentration, and parelectric behaviour leading to a huge dielectric constant at low temperatures. For nanostructured constrictions of PbTe, we have observed 1-dimensional quantization of electron motion at much more impure conditions than in any other system studied so far. This is possibly due to the dielectric screening of Coulomb potential fluctuations produced by various defects usually existing in the solid-state environment. In an external magnetic field, this quantization exhibits a very pronounced spin splitting, already discernible at several kilogauss. This indicates that PbTe nanostructures are very promising as local spin-filtering devices.

Conductance quantization was measured in submicron mesa constrictions, patterned into PbTe/Pb0.92Eu0.08Te quantum wells of thickness 12, 25 and 50nm. The main observation is almost unitary transmission of the one-dimensional quantum modes, despite of significant amount of charged defects in the vicinity of the constrictions. Such an extraordinary result is a consequence of the paraelectric properties of PbTe, namely, the suppression of long-range tails of Coulomb potentials due to the huge dielectric constant.

The role of spin-orbit and s-d exchange interactions is investigated by magnetoresistance (MR) measurements carried out down to 50 mK on ZnO and Zn1-xMnxO with x = 3 and 7% and electron concentration similar to 10(20) cm(-3). The data for ZnO:Al leads to the coupling constant lambda(so) = (4.4 +/- 0.4) x 10(-11) eV cm of the kp hamiltonian for the wurzite structure, H-so = lambda(so)c(s x k). Complex MR of Zn1-xMnxO:Al is interpreted in terms of the influence of the s-d spin-splitting on the disorder-modified electron-electron interactions and precursor effects of magnetism-related localization. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.