Search

The effect of infrared, green, and ultraviolet laser radiation on the electrical resistance of the Ba0.8Sr0.2TiO3/LaMnO3 heterostructure is studied. Illumination increases the resistance in the temperature range of 80–200 K with a characteristic time of ~4–15 s. The negative effect of the photoconductivity is detected under irradiation by infrared, green, and ultraviolet radiation. [ABSTRACT FROM AUTHOR]

For a chiral topological semimetal CoSi we experimentally investigate second-harmonic transverse voltage response to ac electrical current for two (normal and in-plane) orientations of magnetic field. Without magnetic field, the observed response depends quadratically on the longitudinal current, as it should be expected for nonlinear Hall effect in topological semimetals. In the external magnetic field, the second-harmonic Hall voltage shows odd-type dependence on the field direction for both field orientations. In normal field, this sensitivity to the field direction allows to exclude possible contribution of the thermopower effects, so the second-harmonic transverse voltage response indeed originates from the nonlinear Hall effect. In contrast, for the in-plane fields, odd-type field dependence is a clear demonstration of a novel planar nonlinear Hall effect for chiral CoSi semimetal. [ABSTRACT FROM AUTHOR]

We review recent studies of the spin-dependent tunneling transport via an Aharonov–Bohm interferometer (ABI) formed by helical edge states. We focus on the experimentally relevant case of relatively high temperature, T, as compared to level spacing, Δ. The tunneling conductance of helical ABI is structureless in ballistic case but shows sharp periodic antiresonances as a function of magnetic flux with the period in presence of magnetic impurities. The incoming unpolarized electron beam acquires finite polarization after transmission through the helical ABI provided that the edges contain at least one magnetic impurity. The finite polarization appears even in the fully classical regime and is therefore robust to dephasing. There is also a quantum contribution to the polarization, which shows sharp identical resonances as a function of magnetic flux with the same period as conductance. This polarization survives at relatively high temperature. The interferometer can be described in terms of ensemble of flux-tunable qubits giving equal contributions to conductance and spin polarization. Hence, with increasing the temperature number of active qubits participating in the charge and spin transport increases. These features of tunneling helical ABI open a wide avenue for applications in the area of quantum computing. [ABSTRACT FROM AUTHOR]

A two-dimensional semimetal is discovered in the (013) HgTe quantum well with a thickness of d = 14 nm, which is much smaller than those previously studied. It is found that such semimetal is characterized by the same band overlap as the wells with d =18−22 nm having the same orientation, but here the impurity scattering of both electrons and holes is much more pronounced. The electron cyclotron photoresistance is measured as a function of the electron density (Ns) and it is shown that the amplitude of the electron cyclotron photoresistance decreases with decreasing density, and the electron cyclotron photoresistance is not detected at Ns < 5 × 109 cm−2. Thus, the two-dimensional semimetal under study does not exhibit the Ns-independent electron cyclotron photoresistance, which was earlier observed in the two-dimensional semimetal arising near the (100) surface. This is assumingly due to a significantly lower (by more than an order of magnitude) electron mobility in the system under study. [ABSTRACT FROM AUTHOR]

We investigate electron transport along the surface of WTe2 three-dimensional single crystals, which are characterized by the coexistence of the conductivity and ferroelectricity of a Weyl semimetal at room temperature. We find that a nonlinear behavior of the differential resistance of WTe2 is accompanied by a slow relaxation process, which appears as the dependence on the sign of the current change. This observation is confirmed by direct investigation of time-dependent relaxation curves. While strongly nonlinear differential resistance should be expected for zero-gap WTe2, the slow relaxation in transport is very unusual for well-conducting semimetals at room temperature. We establish that a nonmonotonic dependence of the amplitude of the effect on the driving current well corresponds to the known Sawyer–Tower's ferroelectric hysteresis loop. The possibility to induce polarization current by source-drain field variation is unique for WTe2, since it is a direct consequence of the coexistence of ferroelectricity and metallic conduction. This conclusion is also confirmed by gate voltage dependencies, so our results can be understood as a direct demonstration of the ferroelectric behavior of WTe2 in charge transport experiment. [ABSTRACT FROM AUTHOR]

Being encouraged by the interplay between topology, superconductivity and magnetism, we experimentally investigate charge transport through the interface between the Nb superconductor and the time-reversal symmetry breaking Weyl semimetal Co3Sn2S2. In addition to the proximity-induced superconducting gap, we observe prominent subgap zero-bias anomaly. The anomaly demonstrates an unusual robustness to external magnetic fields: its width is absolutely stable up to the critical field of Nb, while its amplitude exhibits a weak non-monotonous variation. As the promising scenario of emergence of the zero-bias anomaly in transport characteristics, we consider the proximity-induced zero-energy Andreev bound states interfaced with the half-metallic Co3Sn2S2 and influenced by the strong spin-orbit coupling and large Zeeman splitting. [ABSTRACT FROM AUTHOR]

Intrinsic Si(111) surfaces passivated with atomic hydrogen are an ideal platform to host two-dimensional electron systems. Traditional methods to probe these surfaces, however, typically involve the placement of dopants and metals directly onto the surface and subsequent high temperature processing, which can be harsh and invasive and lead to surface degradation. Here, we detail a non-invasive gating approach for probing two-dimensional electron systems on intrinsic H-Si(111) surfaces using a silicon-on-insulator (SOI) gating assembly. In this architecture, all harsh device fabrication is performed on a single SOI chip, ensuring that the H-Si(111) surface remains in pristine condition, or as close to the original manufactured intrinsic-Si wafer as possible. To achieve this, we intentionally keep our H-Si(111) surfaces free of any dopants or metals, which are instead placed on the adjacent SOI chip. All electrical components, including Ohmic contacts and accumulation and depletion gates, are housed in the SOI piece. The Ohmic contacts on the SOI piece are brought into physical and electrical contact with the pristine H-Si(111) piece after being van der Waals bonded at room temperature, while all gates on the SOI piece are separated from the H-Si(111) surface by vacuum. Architecture details, baseline operation tests, and 77 K device characterization measurements will be discussed, as well as the implications of going beyond H-Si(111) surfaces and using our device architecture to facilitate transport measurements on halogen-terminated Si surfaces. [ABSTRACT FROM AUTHOR]

Dichalcogenides are known from almost all transition metals. The representatives of this class of compounds show a number of interesting physical and chemical properties depending on their constituent transition metal and crystal structure, which makes them interesting for basic studies and applications in high‐end electronics, spintronics, optoelectronics, energy storage, flexible electronics, DNA sequencing and personalized medicine to this day. Many of these properties and effects can only be investigated on chemically and crystallographically pure samples ‐ usually on single crystals. The vast majority of these compounds can be crystallized using chemical vapour transport. However crystallization from the melt is also possible in a considerable number of compounds, including the frequently used self‐flux technique. For several compounds the crystallization from different solvents or solvent mixtures by means of solvothermal or hydrothermal synthesis is described. [ABSTRACT FROM AUTHOR]

We study conductivity of strongly disordered amorphous antimony films under high bias voltages. We observe non-linear current-voltage characteristic, where the conductivity value at zero bias is one of two distinct values, being determined by the sign of previously applied voltage. Relaxation curves demonstrate high stability of these conductivity values on a large timescale. Investigations of the antimony film structure allows to determine the percolation character of electron transport in strongly disordered films. We connect the memory effect in conductivity with modification of the percolation pattern due to recharging of some film regions at high bias voltages. [ABSTRACT FROM AUTHOR]

Fundamentals of fermion-condensation physics are outlined. Fermion condensation is a phase transition occurring in a strongly correlated Fermi system upon a topological reconstruction of the Landau ground state and leading to the formation of a fermion condensate that possesses the dispersionless single-particle spectrum in the momentum-space region adjacent to the Fermi surface and, accordingly, an anomalously enhanced density of single-particle states. A original method is developed for solving the set of nonlinear integral equations of fermion-condensation theory. This method makes it possible to analyze the problem of quantum chaos in strongly interacting multifermion systems. The computational technique used is demonstrated by applying it to superdense quark–gluon plasma, where the structure of exchange quark–quark interaction is well known. In electron systems featuring a fermion condensate, the magnitude of the gap appearing in the single-particle spectrum owing to Cooper pairing is shown to be much larger than that in Bardeen–Cooper–Schrieffer (BCS) theory. This explains both a high superconducting-transition temperature and, with allowance for crystal-lattice symmetry, the -wave pairing-gap structure observed in cuprates. It is found that, in addition to the BCS gap , the spectrum of single-particle excitations of superconducting systems where a fermion condensate is present develops a nonsuperconducting gap that owes its existence to the interaction of condensate particles with normal quasiparticles residing outside the condensate region in momentum space. The question of whether these results are pertinent to the two-gap structure recently unearthed in the excitation spectrum of cuprates upon analyzing ARPES data is addressed. [ABSTRACT FROM AUTHOR]

We experimentally investigate a nonlinear Hall effect for three-dimensional WTe2 and Cd3As2 single crystals, representing Weyl and Dirac semimetals, respectively. We observe finite second-harmonic Hall voltage, which depends quadratically on the longitudinal current in zero magnetic field, as it has been predicted theoretically. We demonstrate that second-harmonic Hall voltage shows odd-type dependence on the direction of the magnetic field, which is a strong argument in favor of current-magnetization effects. In contrast, one order of magnitude higher thermopower signal is independent of the magnetic field direction. Thus, the magnetic field dependence allows distinguishing the nonlinear Hall effect from a thermoelectric response. [ABSTRACT FROM AUTHOR]

We experimentally investigate spin-polarized transport between a ferromagnetic Ni electrode and a surface of Weyl semimetal, realized in a thick WTe2 single crystal. For highly transparent Ni-WTe2 planar junctions, we observe non-Ohmic dV/dI (I) behavior with an overall increase in differential resistance dV/dI with current bias, which is accomplished by peaks and dips structures. This behavior is inconsistent with trivial interface scattering, but it is well known for spin-polarized transport with magnon emission. Thus, we interpret the experimental results in terms of spin wave excitation in spin textures in the WTe2 topological surface states, which is supported by the obtained magnetic field and temperature dependences of dV/dI (I). [ABSTRACT FROM AUTHOR]

We experimentally study electron transport between two superconducting indium leads, coupled to a single WTe2 crystal, which is a three-dimensional Weyl semimetal. We demonstrate Josephson current in 5 μm long In-WTe2-In junctions, as confirmed by the observation of integer (1, 2, 3) and fractional (1/3, 1/2, 2/3) Shapiro steps under microwave irradiation. The demonstration of the fractional a.c. Josephson effect indicates the multivalued character of the current-phase relationship, which we connect with the Weyl topological surface states contribution to the Josephson current. In contrast to topological insulators and Dirac semimetals, we do not observe periodicity in the a.c. Josephson effect for WTe2 at different frequencies and power, which might reflect the chiral character of the Fermi arc surface states in a Weyl semimetal. [ABSTRACT FROM AUTHOR]

The thermopower in a two-dimensional semimetal existing in HgTe quantum wells 18-21 nm thick has been studied experimentally and theoretically for the first time. It has been found theoretically and experimentally that the thermopower has two components—diffusion and phonon-drag—and that the second component is several times larger than the first. It has been concluded that the electron-hole scattering plays an important role in both mechanisms of the thermopower. [ABSTRACT FROM AUTHOR]

We experimentally study electron transport between two superconducting indium leads, coupled to the WTe2 crystal surface. WTe2 is characterized by presence of Fermi arc surface states, as a predicted type-II Weyl semimetal candidate. We demonstrate Josephson current in unprecedentedly long 5 µm In-WTe2-In junctions, which is confirmed by I-V curves evolution with temperature and magnetic field. The Josephson current is mostly carried by the topological surface states, which we demonstrate in a double-slit superconducting quantum interference device geometry, realized by coupling the opposite WTe2 crystal surfaces. [ABSTRACT FROM AUTHOR]

We observe an unusual behavior of the low-temperature magnetoresistance of the high-mobility two-dimensional electron gas in InGaAs/InAlAs quantum wells in weak perpendicular magnetic fields. The observed magnetoresistance is qualitatively similar to that expected for the weak localization and antilocalization but its quantity exceeds significantly the scale of the quantum corrections. The calculations show that the obtained data can be explained by the classical effects in electron motion along the open orbits in a quasiperiodic potential relief manifested by the presence of ridges on the quantum well surface. [ABSTRACT FROM AUTHOR]

We discuss the non-equilibrium properties of integer channels in nonuniform 2D electron (hole) systems in the presence of a strong magnetic field. The results are applied to a qualitative explanation of the Corbino disk current-voltage characteristics (IVC) in the quantum Hall effect (QHE) regime. Special consideration is paid to the so-called “QHE breakdown” effect, which is readily observed in both the Hall bar and Corbino geometries of the tested cells. The QHE breakdown is especially evident in the Corbino samples, allowing for a more in-depth study of these effects. [ABSTRACT FROM AUTHOR]

We consider the effects of electron scattering off a quantum magnetic impurity on the current-voltage characteristics of the helical edge of a two-dimensional topological insulator. We compute the backscattering contribution to the current along the edge for a general form of the exchange interaction matrix and arbitrary value of the magnetic impurity spin. We find that the differential conductance may exhibit a nonmonotonic dependence on the voltage with several extrema. [ABSTRACT FROM AUTHOR]

We investigate charge transport through the junction between a niobium superconductor and the edge of a two-dimensional electron-hole bilayer, realized in an InAs/GaSb double quantum well. For the transparent interface with a superconductor, we demonstrate that the junction resistance is determined by the interlayer charge transfer near the interface. From an analysis of experimental I−V curves, we conclude that the proximity-induced superconductivity efficiently couples electron and hole layers at low currents. The critical current demonstrates periodic dependence on the in-plane magnetic field, while it is monotonic for the field that is normal to the bilayer plane. [ABSTRACT FROM AUTHOR]

Investigations of topological insulators, which are two- and three-dimensional systems with a gap in the bulk spectrum and topologically protected gapless edge states, are of considerable fundamental interest at present. The experiments confirming the presence of the edge states in two-dimensional systems with inverted bands and problems of determining the nature of such states in these experiments are reviewed. Special attention is focused on spin-sensitive experiments since the topological edge states have a nontrivial spin structure. [ABSTRACT FROM AUTHOR]

The terahertz resistive response of a two-dimensional topological insulator in a HgTe quantum well in the quasiballistic transport regime is studied. The photoresistance appearing only near the charge neutrality point is detected. The application of the magnetic field up to 4 T in the plane of the quantum well results in an increase in the photoresistance in the peak and in the expansion of the region near the charge neutrality point where it exists. The reported results imply that the observed photoresistance is due to transitions involving edge dispersion branches of the two-dimensional topological insulator. [ABSTRACT FROM AUTHOR]

We experimentally investigate spin-polarized electron transport between a permalloy ferromagnet and the edge of a two-dimensional electron system with band inversion, realized in a narrow, 8 nm wide, HgTe quantum well. In zero magnetic field, we observe strong asymmetry of the edge potential distribution with respect to the ferromagnetic ground lead. This result indicates that the helical edge channel, specific for the structures with band inversion even at the conductive bulk, is strongly coupled to the ferromagnetic side contact, possibly due to the effects of proximity magnetization. This allows selective and spin-sensitive contacting of helical edge states. [ABSTRACT FROM AUTHOR]

We experimentally investigate transport through the side junction between a niobium superconductor and the mesa edge of a two-dimensional system, realized in an InAs/GaSb double quantum well with band inversion. We demonstrate, that different transport regimes can be achieved by variation of the mesa step. We observe anomalous behavior of Andreev reflection within a finite low-bias interval, which is invariant for both transport regimes. We connect this behavior with the transition from retro-(at low biases) to specular (at high ones) Andreev reflection channels in an InAs/GaSb double quantum well with band inversion. [ABSTRACT FROM AUTHOR]

Precise measurements of the magnetic-field and gate-voltage dependences of the capacitance of a field-effect transistor with an electron system in a wide GaAs quantum well have been carried out. It has been found that the capacitance minima caused by the gaps in the Landau spectrum of the electron system become anomalously wide when two size-quantization subbands are occupied. The effect is explained by retention of the chemical potential in the gap between the Landau levels of one of the subbands owing to redistribution of electrons between the subbands under a change in the magnetic field. The calculation taking into account this redistribution has been performed in a model of the electron system formed by two two-dimensional electron layers. The calculation results describe both the wide capacitance features and the observed disappearance of certain quantum Hall effect states. [ABSTRACT FROM AUTHOR]

The current flow along the boundary of graphene stripes in a perpendicular magnetic field is studied theoretically by the nonequilibrium Green's function method. In the case of specular reflections at the boundary, the Hall resistance shows equidistant peaks, which are due to classical cyclotron motion. When the strength of the magnetic field is increased, anomalous resistance oscillations are observed, similar to those found in a nonrelativistic 2D electron gas [New. J. Phys. 15:113047 (2013)]. Using a simplified model, which allows to solve the Dirac equation analytically, the oscillations are explained by the interference between the occupied edge states causing beatings in the Hall resistance. A rule of thumb is given for the experimental observability. Furthermore, the local current flow in graphene is affected significantly by the boundary geometry. A finite edge current flows on armchair edges, while the current on zigzag edges vanishes completely. The quantum Hall staircase can be observed in the case of diffusive boundary scattering. The number of spatially separated edge channels in the local current equals the number of occupied Landau levels. The edge channels in the local density of states are smeared out but can be made visible if only a subset of the carbon atoms is taken into account. [ABSTRACT FROM AUTHOR]

Transport measurements at liquid helium temperatures were done on a number of BiTeSe samples with thicknesses ranging from 30 to 200 μm in order to detect surface states. In each sample we observed Shubnikov-de Haas (SdH) oscillations and sublinear dependence of off-diagonal component of magnetoresistance tensor on magnetic field. The periods of SdH oscillations in inverse magnetic field were found to be the same within 15%. The positions of SdH oscillations are determined by the normal to surface component of magnetic field. We found that the measured conductivity can be well described by a model with two groups of electrons, 2D and 3D. The conductivity of 2D electrons was found to be relatively weakly varying from sample to sample and not depending on thickness in a systematic manner. This behavior can be explained only by their localization on the surface. Comparison of the results of magnetotransport measurements with our scanning tunneling spectroscopy results on atomically smooth BiTeSe surface in ultrahigh vacuum led us to conclude that the surface electrons are separated from the bulk electrons by a depletion layer approximately 100 nm thick. This effect could provide the dominant contribution of surface electrons to conductivity in samples with thicknesses less than 200 nm. [ABSTRACT FROM AUTHOR]

We show that the disappearance of the chemical potential jumps over the range of perpendicular magnetic fields at fixed integer filling factor in a double quantum well with a tunnel barrier is caused by the interactioninduced level merging. The distribution function in the merging regime is special in that the probability to find an electron with energy equal to the chemical potential is different for the two merged levels. [ABSTRACT FROM AUTHOR]

We experimentally investigate spin-polarized electron transport between two ferromagnetic contacts, placed at the edge of a two-dimensional electron system with band inversion. The system is realized in a narrow (8 nm) HgTe quantum well, the ferromagnetic side contacts are formed from a premagnetized permalloy film. In zero magnetic field, we find a significant edge current contribution to the transport between two ferromagnetic contacts. We experimentally demonstrate that this transport is sensitive to the mutual orientation of the magnetization directions of two 200 µm-spaced ferromagnetic leads. This is a direct experimental evidence on the spin-coherent edge transport over the macroscopic distances. Thus, the spin is extremely robust at the edge of a two-dimensional electron system with band inversion, confirming the helical spin-resolved nature of edge currents. [ABSTRACT FROM AUTHOR]

We investigate the current noise in HgTe-based quantum wells with an inverted band structure in the regime of disordered edge transport. Consistent with previous experiments, the edge resistance strongly exceeds h/ e and weakly depends on the temperature. The shot noise is well below the Poissonian value and characterized by the Fano factor with gate voltage and sample-to-sample variations in the range 0.1 < F < 0.3. Given the fact that our devices are shorter than the most pessimistic estimate of the ballistic dephasing length, these observations exclude the possibility of one-dimensional helical edge transport. Instead, we suggest that a disordered multi-mode conduction is responsible for the edge transport in our experiment. [ABSTRACT FROM AUTHOR]

We report the observation of an electron gas in a SiGe/Si/SiGe quantum well with maximum mobility up to 240 m²/Vs, which is noticeably higher than previously reported results in siliconbased structures. Using SiO, rather than Al2O3, as an insulator, we obtain strongly reduced threshold voltages close to zero. In addition to the predominantly small-angle scattering well known in the high-mobility heterostructures, the observed linear temperature dependence of the conductivity reveals the presence of a short-range random potential. [ABSTRACT FROM AUTHOR]

Here, we report on a transparent method to characterize individual layers in a double-layer electron system, which forms in a wide quantum well, and to determine their electron densities. The technique relies on the simultaneous measurement of the capacitances between the electron system and gates located on either side of the well. Modifications to the electron wave function due to the population of the second subband and the appearance of an additional electron layer can be detected. The magnetic field dependence of these capacitances is dominated by quantum corrections caused by the occupation of Landau levels in the nearest electron layer. The technique should be equally applicable to other implementations of a double layer electron system. [ABSTRACT FROM AUTHOR]

Recent theoretical and experimental attempts have been successful in finding magnetic Weyl semimetal phases, which show nodal‐point structure in the electronic bands as well as magnetic orders. Beyond uniform ferromagnetic or antiferromagnetic orders, nonuniform magnetic textures, such as domain walls and skyrmions, enrich the properties of the Weyl electrons even more in such materials. Here, a topical review on interplay between Weyl electrons and magnetic textures in those magnetic Weyl semimetals is given. The basics of magnetic textures in nontopological magnetic metals are reviewed first, and then the effect of magnetic textures in Weyl semimetals is discussed, regarding the recent theoretical and experimental progress therein. The idea of the fictitious "axial gauge fields" is pointed out, which effectively describes the effect of magnetic textures on the Weyl electrons and can well account for the properties of the electrons localized around magnetic domain walls. [ABSTRACT FROM AUTHOR]

Excitation of a topological insulator by a high‐frequency electric field of a laser radiation leads to a dc electric current in the helical edge channel whose direction and magnitude are sensitive to the radiation polarization and depend on the physical properties of the edge. An overview of theoretical and experimental studies of such edge photoelectric effects in 2D topological insulators based on semiconductor quantum wells is presented. First, a phenomenological description is given of edge photocurrents, which may originate from the photogalvanic effects or the photon drag effects, for edges of all possible symmetry. Then, microscopic mechanisms of photocurrent generation for different types of optical transitions involving helical edge states are discussed. They include direct and indirect optical transitions within the edge channel and edge‐to‐bulk optical transitions. [ABSTRACT FROM AUTHOR]

The characteristics of a charge-sensitive infrared phototransistor (CSIP) based on a GaAs/AlGaAs multiple quantum-well (QW) structure are studied under a magnetic field. In the CSIP, the upper QWs serve as a floating gate that is charged by photoexcitation. The photoinduced charges are detected using the resistance of the lowest QW conducting channel. The conducting channel exhibits the integer quantum Hall effect (QHE) in a perpendicular high magnetic field, yielding the magnetic field dependence of the terahertz (THz) response ΔR. We found two different features of ΔR. One is that ΔR switches sign across the QHE plateau, which is explained simply by an increased electron density in the conducting channel. The other feature is observed as an enhanced positive ΔR when a potential barrier is formed in the conducting channel. The latter mechanism can be interpreted as the promotion of edge/bulk scattering due to photoinduced charges. These findings suggest ways to enhance the THz response by using magnetic fields and potential barriers. [ABSTRACT FROM AUTHOR]