Quantum spin Hall insulator with a large bandgap, Dirac fermions, and bilayer graphene analog.

The search for room temperature quantum spin Hall insulators (QSHIs) based on widely available materials and a controlled manufacturing process is one of the major challenges of today's topological physics. We propose a new class of semiconductor systems based on multilayer broken-gap quantum wells, in which the QSHI gap reaches 60 meV and remains insensitive to temperature. Depending on their layer thicknesses and geometry, these novel structures also host a graphene-like phase and a bilayer graphene analog.

Evolution of Metastable Defects and Its Effect on the Electronic Properties of MoS Films.

We report on structural and electronic properties of defects in chemical vapor-deposited monolayer and few-layer MoS films. Scanning tunneling microscopy, Kelvin probe force microscopy, and transmission electron microscopy were used to obtain high resolution images and quantitative measurements of the local density of states, work function and nature of defects in MoS films. We track the evolution of defects that are formed under heating and electron beam irradiation.

Laser-Rewriteable Ferromagnetism at Thin-Film Surfaces.

Manipulation of magnetism using laser light is considered as a key to the advancement of data storage technologies. Until now, most approaches seek to optically switch the direction of magnetization rather than to reversibly manipulate the ferromagnetism itself. Here, we use ∼100 fs laser pulses to reversibly switch ferromagnetic ordering on and off by exploiting a chemical order-disorder phase transition in FeAl, from the B2 to the A2 structure and vice versa.

Temperature-Induced Topological Phase Transition in HgTe Quantum Wells.

We report a direct observation of temperature-induced topological phase transition between the trivial and topological insulator states in an HgTe quantum well. By using a gated Hall bar device, we measure and represent Landau levels in fan charts at different temperatures, and we follow the temperature evolution of a peculiar pair of "zero-mode" Landau levels, which split from the edge of electronlike and holelike subbands. Their crossing at a critical magnetic field B_{c} is a characteristic of inverted band structure in the quantum well.

Spin current and second harmonic generation in non-collinear magnetic systems: the hydrodynamic model.

We report a theoretical study of the second harmonic generation in a noncollinearly magnetized conductive medium with equilibrium spin current. The hydrodynamic model is used to unravel the mechanism of a novel effect of the double frequency signal generation that is attributed to the spin current. According to our calculations, this second harmonic response appears due to the 'non-adiabatic' spin polarization of the conduction electrons induced by the oscillations in the non-uniform magnetization forced by the electric field of the electromagnetic wave.

Quantitative characterization of semiconductor structures with a scanning microwave microscope.

In this work, our earlier method for measuring resistance R of semiconductor films with a near-field scanning microwave microscope [A. N. Reznik and S.

Quantum Dot Emission Driven by Mie Resonances in Silicon Nanostructures.

Resonant dielectric nanostructures represent a promising platform for light manipulation at the nanoscale. In this paper, we describe an active photonic system based on Ge(Si) quantum dots coupled to silicon nanodisks. We show that Mie resonances govern the enhancement of the photoluminescent signal from embedded quantum dots due to a good spatial overlap of the emitter position with the electric field of Mie modes.

Magnetic Force Microscopy of Nanostructured Co/Pt Multilayer Films with Perpendicular Magnetization.

We present the results of magnetic force microscopy investigations of domain structures in multilayer [Co (0.5 nm)/Pt (1 nm)]₅ thin film structures (denoted hereafter as Co/Pt) modified by additional Co capping layers and by ion irradiation. It is demonstrated that a Co capping layer essentially changes the domain structure and decreases the threshold of magnetization reversal, due to the formation of noncollinear magnetization in Co/Pt.

Calibration of multi-layered probes with low/high magnetic moments.

We present a comprehensive method for visualisation and quantification of the magnetic stray field of magnetic force microscopy (MFM) probes, applied to the particular case of custom-made multi-layered probes with controllable high/low magnetic moment states. The probes consist of two decoupled magnetic layers separated by a non-magnetic interlayer, which results in four stable magnetic states: ±ferromagnetic (FM) and ±antiferromagnetic (A-FM). Direct visualisation of the stray field surrounding the probe apex using electron holography convincingly demonstrates a striking difference in the spatial distribution and strength of the magnetic flux in FM and A-FM states.

Sensitization of NIR luminescence of Yb by Zn chromophores in heterometallic complexes with a bridging Schiff-base ligand.

Herein, complexes [ZnL] (1), {(HO)Zn(μ-L)Yb[OCH(CF)]} (2), {[(CF)HCO]Zn(μ-L)Yb[OCH(CF)](μ-OH)} (3), and [(HO)Ln(L)] (Ln = Yb (4) and Gd (5)) containing a bridging Schiff-base ligand (HL = N,N'-bis(3-methoxy salicylidene)phenylene-1,2-diamine) were synthesized. The compounds 1-4 were structurally characterized. The ytterbium derivatives 2-4 exhibited bright NIR metal-centred photoluminescence (PL) of Yb ion under one- (λ = 380 nm) and two-photon (λ = 750 nm) excitation.

Switchable bi-stable multilayer magnetic probes for imaging of soft magnetic structures.

We present the use of custom-made multilayer (ML) magnetic probes in magnetic force microscopy (MFM) for imaging soft magnetic structures, i.e. nickel submicron disks of different dimensions.

Microscopic theory of the Coulomb based exchange coupling in magnetic tunnel junctions.

We study interlayer exchange coupling based on the many-body Coulomb interaction between conduction electrons in magnetic tunnel junction. This mechanism complements the known interaction between magnetic layers based on virtual electron hopping (or spin currents). We find that these two mechanisms have different behavior on system parameters.

Influence of the Coulomb interaction on the exchange coupling in granular magnets.

We develop a theory of the exchange interaction between ferromagnetic (FM) metallic grains embedded into insulating matrix by taking into account the Coulomb blockade effects. For bulk ferromagnets separated by the insulating layer the exchange interaction strongly depends on the height and thickness of the tunneling barrier created by the insulator. We show that for FM grains embedded into insulating matrix the exchange coupling additionally depends on the dielectric properties of this matrix due to the Coulomb blockade effects.

Spontaneous Currents in Superconducting Systems with Strong Spin-Orbit Coupling.

We show that Rashba spin-orbit coupling at the interface between a superconductor and a ferromagnet should produce a spontaneous current in the atomic thickness region near the interface. This current is counterbalanced by the superconducting screening current flowing in the region of the width of the London penetration depth near the interface. Such a current-carrying state creates a magnetic field near the superconductor surface, generates a stray magnetic field outside the sample edges, changes the slope of the temperature dependence of the critical field H_{c3}, and may generate the spontaneous Abrikosov vortices near the interface.

LMCT facilitated room temperature phosphorescence and energy transfer in substituted thiophenolates of Gd and Yb.

To obtain luminescent lanthanide complexes with a low energy LMCT state the 2-(2'-mercaptophenyl)benzothiazolates, Ln(SSN), and 2-(2'-mercaptophenyl)benzoxazolates, Ln(OSN) (Ln = Gd, Yb), were synthesized by the reaction of amides Ln[N(SiMe)] with respective thiophenols. Ytterbium complexes were structurally characterized by X-ray diffraction analysis. Cyclic voltammetry revealed that the deprotonated mercaptophenyl ligands have significantly lower oxidation potentials than their phenoxy analogues and some β-diketones.

Investigation of terahertz radiation influence on rat glial cells.

We studied an influence of continuous terahertz (THz) radiation (0.12 - 0.18 THz, average power density of 3.

Characterization of the cleaved edge cross section of the heterostructures with GaMnAs layer by the confocal micro-Raman spectroscopy.

Confocal micro-Raman spectroscopy was used to measure cross-section linescans of the cleaved edge of heterostructures involving a GaMnAs layer. The investigations revealed a shift of the TO mode in the compressed GaMnAs layer to high frequencies relative to the TO GaAs mode in the substrate and to low frequencies in the tensile GaMnAs layers. These results are in agreement with the different manifestations of the anomalous Hall effect in the GaMnAs layers, with either compressive or tensile strains.

Temperature-driven massless Kane fermions in HgCdTe crystals.

It has recently been shown that electronic states in bulk gapless HgCdTe offer another realization of pseudo-relativistic three-dimensional particles in condensed matter systems. These single valley relativistic states, massless Kane fermions, cannot be described by any other relativistic particles. Furthermore, the HgCdTe band structure can be continuously tailored by modifying cadmium content or temperature.

Giant Mesoscopic Fluctuations and Long-Range Superconducting Correlations in Superconductor-Ferromagnet Structures.

The fluctuating superconducting correlations emerging in dirty hybrid structures under the conditions of the strong proximity effect are demonstrated to affect the validity range of the widely used formalism of Usadel equations at mesoscopic scales. In superconductor-ferromagnet structures these giant mesoscopic fluctuations originating from the interference effects for the Cooper pair wave function in the presence of the exchange field can be responsible for an anomalously slow decay of superconducting correlations in a ferromagnet even when the noncollinear and spin-orbit effects are negligible. The resulting sample-to-sample fluctuations of the Josephson current in superconductor-ferromagnetic-superconductor junctions and the local density of states in superconductor-ferromagnetic hybrid structures can provide an explanation of the long-range proximity phenomena observed in mesoscopic samples with collinear magnetization.

Phase transitions in two tunnel-coupled HgTe quantum wells: Bilayer graphene analogy and beyond.

HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the "single-valley" analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier.

Single Quantum Level Electron Turnstile.

We report on the realization of a single-electron source, where current is transported through a single-level quantum dot (Q) tunnel coupled to two superconducting leads (S). When driven with an ac gate voltage, the experiment demonstrates electron turnstile operation. Compared to the more conventional superconductor-normal-metal-superconductor turnstile, our superconductor-quantum-dot-superconductor device presents a number of novel properties, including higher immunity to the unavoidable presence of nonequilibrium quasiparticles in superconducting leads.

Features of spectral properties of Sm(3+) complexes with dithia- and diselenophosphinate ligands.

The samarium complexes Sm(S2PPh2)3(THF)2 (1) and Sm(Se2PPh2)3(THF)2 (2) with soft-donor dithia- and diselenophosphinate ligands were synthesized and their photophysical properties were studied in detail. Both complexes displayed the metal-centered photoluminescence (PL) in visible and NIR regions corresponding to (4)G5/2→(6)HJ (J=5/2, 7/2, 9/2, 11/2, 13/2, 15/2), (6)FJ (J=1/2, 3/2, 5/2, 7/2, 9/2, 11/2) f-f transitions of Sm(3+). Luminescence decay curves exhibit an initial short build-up region and can be described by double or triple exponential function owing to multiphonon relaxation from the (4)F3/2 energy level to the (4)G5/2 one and reversible energy transfer from the Sm(3+) excited states to the triplet ((3)T1) state of phosphinate ligand.

Structural and electronic properties of epitaxial multilayer h-BN on Ni(111) for spintronics applications.

Hexagonal boron nitride (h-BN) is a promising material for implementation in spintronics due to a large band gap, low spin-orbit coupling, and a small lattice mismatch to graphene and to close-packed surfaces of fcc-Ni(111) and hcp-Co(0001). Epitaxial deposition of h-BN on ferromagnetic metals is aimed at small interface scattering of charge and spin carriers. We report on the controlled growth of h-BN/Ni(111) by means of molecular beam epitaxy (MBE).

Tunable quasiparticle trapping in Meissner and vortex states of mesoscopic superconductors.

Nowadays, superconductors serve in numerous applications, from high-field magnets to ultrasensitive detectors of radiation. Mesoscopic superconducting devices, referring to those with nanoscale dimensions, are in a special position as they are easily driven out of equilibrium under typical operating conditions. The out-of-equilibrium superconductors are characterized by non-equilibrium quasiparticles.

On-Chip Maxwell's Demon as an Information-Powered Refrigerator.

We present an experimental realization of an autonomous Maxwell's demon, which extracts microscopic information from a system and reduces its entropy by applying feedback. It is based on two capacitively coupled single-electron devices, both integrated on the same electronic circuit. This setup allows a detailed analysis of the thermodynamics of both the demon and the system as well as their mutual information exchange.