Room temperature electroluminescence from mechanically formed van der Waals III–VI homojunctions and heterojunctions

Room temperature electroluminescence from semiconductor junctions is demonstrated. The junctions are fabricated by the exfoliation and direct mechanical adhesion of InSe and GaSe van der Waals layered crystals. Homojunction diodes formed from layers of p- and n-type InSe exhibit electroluminescence at energies close to the bandgap energy of InSe (Eg= 1.26 eV). In contrast, heterojunction diodes formed by combining layers of p-type GaSe and n-type InSe emit photons at lower energies, which is attributed to the generation of spatially indirect excitons and a staggered valence band lineup for the holes at the GaSe/InSe interface. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct-bandgap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near-infrared to the visible spectrum

Harmonic Content of Strain-induced Potential Modulation in Unidirectional Lateral Superlattices

Detailed analysis of the commensurability oscillation (CO) has been performed on unidirectional lateral superlattices with periods ranging from a=92 to 184 nm. Fourier analysis reveals the second (and the third) harmonics along with the fundamental oscillation for a>=138 nm (184 nm) at low-enough temperature, evincing the presence of corresponding harmonics in the profile of the potential modulation. The harmonics manifest themselves in CO with demagnified amplitude due to the low-pass filtering action of the thermal damping factor; with a suitable consideration of the damping effect, the harmonics of the modulation potential are found to have the amplitudes V_2 and V_3 up to roughly 30% of that of the fundamental component V_1, despite the small ratio of the period a to the depth d = 99 nm of the two-dimensional electron gas (2DEG) from the surface. The dependence of V_n on a indicates that the fundamental component originates at the surface, while the higher harmonics arise from the effect of the strain that penetrates down into subsurface. The manipulation of high harmonics thus provides a useful technique to introduce small length-scale modulation into high-mobility 2DEGs located deep inside the wafer.Comment: 9 pages, 5 figure

Nonequilibrium dynamics of fully frustrated Ising models at T=0

We consider two fully frustrated Ising models: the antiferromagnetic triangular model in a field of strength, $h=H T k_B$, as well as the Villain model on the square lattice. After a quench from a disordered initial state to T=0 we study the nonequilibrium dynamics of both models by Monte Carlo simulations. In a finite system of linear size, $L$, we define and measure sample dependent "first passage time", $t_r$, which is the number of Monte Carlo steps until the energy is relaxed to the ground-state value. The distribution of $t_r$, in particular its mean value, $$, is shown to obey the scaling relation, $\sim L^2 \ln(L/L_0)$, for both models. Scaling of the autocorrelation function of the antiferromagnetic triangular model is shown to involve logarithmic corrections, both at H=0 and at the field-induced Kosterlitz-Thouless transition, however the autocorrelation exponent is found to be $H$ dependent.Comment: 7 pages, 8 figure

Weiss Oscillations in Surface Acoustic Wave Propagation

The interaction of a surface acoustic wave (SAW) with a a two-dimensional electron gas in a periodic electric potential and a classical magnetic field is considered. We calculate the attenuation of the SAW and its velocity change and show that these quantities exhibit Weiss oscillations.Comment: 4 pages REVTEX, 2 figures included as eps file

Cooperative amyloid fibre binding and disassembly by the Hsp70 disaggregase

Although amyloid fibres are highly stable protein aggregates, a specific combination of human Hsp70 system chaperones can disassemble them, including fibres formed of α-synuclein, huntingtin, or Tau. Disaggregation requires the ATPase activity of the constitutively expressed Hsp70 family member, Hsc70, together with the J domain protein DNAJB1 and the nucleotide exchange factor Apg2. Clustering of Hsc70 on the fibrils appears to be necessary for disassembly. Here we use atomic force microscopy to show that segments of in vitro assembled α-synuclein fibrils are first coated with chaperones and then undergo bursts of rapid, unidirectional disassembly. Cryo-electron tomography and total internal reflection fluorescence microscopy reveal fibrils with regions of densely bound chaperones, preferentially at one end of the fibre. Sub-stoichiometric amounts of Apg2 relative to Hsc70 dramatically increase recruitment of Hsc70 to the fibres, creating localised active zones that then undergo rapid disassembly at a rate of ~ 4 subunits per second. The observed unidirectional bursts of Hsc70 loading and unravelling may be explained by differences between the two ends of the polar fibre structure

Magnetoresistance of a two-dimensional electron gas with spatially periodic lateral modulations: Exact consequences of Boltzmann's equation

On the basis of Boltzmann's equation, and including anisotropic scattering in the collision operator, we investigate the effect of one-dimensional superlattices on two-dimensional electron systems. In addition to superlattices defined by static electric and magnetic fields, we consider mobility superlattices describing a spatially modulated density of scattering centers. We prove that magnetic and electric superlattices in $x$-direction affect only the resistivity component $\rho_{xx}$ if the mobility is homogeneous, whereas a mobility lattice in $x$-direction in the absence of electric and magnetic modulations affects only $\rho_{yy}$. Solving Boltzmann's equation numerically, we calculate the positive magnetoresistance in weak magnetic fields and the Weiss oscillations in stronger fields within a unified approach.Comment: submitted to PR

Magnetotunneling as a Probe of Luttinger-Liquid Behavior

A novel method for detecting Luttinger-liquid behavior is proposed. The idea is to measure the tunneling conductance between a quantum wire and a parallel two-dimensional electron system as a function of both the potential difference between them, $V$, and an in-plane magnetic field, $B$. We show that the two-parameter dependence on $B$ and $V$ allows for a determination of the characteristic dependence on wave vector $q$ and frequency $\omega$ of the {\it spectral function}, $A_{\rm LL}(q,\omega)$, of the quantum wire. In particular, the separation of spin and charge in the Luttinger liquid should manifest itself as singularities in the $I$-$V$-characteristic. The experimental feasibility of the proposal is discussed.Comment: Accepted for publication in Phys. Rev. Let

Planar cyclotron motion in unidirectional superlattices defined by strong magnetic and electric fields: Traces of classical orbits in the energy spectrum

We compare the quantum and the classical description of the two-dimensional motion of electrons subjected to a perpendicular magnetic field and a one-dimensional lateral superlattice defined by spatially periodic magnetic and electric fields of large amplitudes. We explain in detail the complicated energy spectra, consisting of superimposed branches of strong and of weak dispersion, by the correspondence between the respective eigenstates and the ``channeled'' and ``drifting'' orbits of the classical description.Comment: 11 pages, 11 figures, to appear in Physical Review

Absence of long-range ordered reconstruction on the GaAs(311)A surface

We have investigated the decapped GaAs(311)A surface using both scanning tunneling microscopy and synchrotron-radiation photoemission. While our data are in broad agreement with the structural model of GaAs(311)A proposed in a recent study [Wassermeier et al., Phys. Rev. B 51, 14 721 (1995)], we find considerable differences in the surface order. In particular, the As dimer rows are unbroken over much shorter length scales and are highly kinked. We observe a correspondingly lower degree of anisotropy in the surface roughness than that previously reported. An (8×1) reconstruction was not observed. An analysis of As 3d and Ga 3d core-level photoemission spectra suggests that surface As atoms are in only one bonding configuration while surface Ga adopts two different bonding states. We discuss possible origins for the core-level spectra surface components

Ligand-induced control of photoconductive gain and doping in a hybrid graphene-quantum dot transistor

In graphene devices decorated with a layer of near-infrared colloidal PbS quantum dots (QDs), the choice of the QD capping ligands and the integrity of the QD layer have a strong influence on the doping, carrier mobility, and photoresponse. By using short (<1 nm) capping ligands, the photoresponsivity of the graphene devices is enhanced up to 109 A W−1